Add Paddle webhook ingestion foundation

.dockerignore

@@ -0,0 +1,18 @@
+.git
+.DS_Store
+**/node_modules
+**/dist
+**/build
+**/.build
+**/DerivedData
+**/.derivedData
+**/coverage
+**/.next
+**/.cache
+**/*.log
+**/.env
+backend/.env
+runtime
+apps/**/build
+packages/apple/**/build
+packages/apple/**/.build

README.md

@@ -2,7 +2,9 @@
 
 Velody is a private music ecosystem for personal use across `macOS`, `iPhone`, and a VPS backend.
 
-This repository currently contains the execution foundation for Phase 1:
+The repository now includes a working backend plus Apple clients for local library upload, remote playback, iPhone sync, offline resilience, account foundation work, and early billing groundwork.
+
+## Repository Layout
 
 - `apps/apple` for the native Apple apps workspace
 - `packages/apple` for shared Swift packages
@@ -19,12 +21,12 @@ This repository currently contains the execution foundation for Phase 1:
 - npm 10+
 - Docker Desktop
 
-## Local Development
+## Local Backend Setup
 
 1. Copy the example environment file:
 
 ```bash
-cp .env.example .env
+cp backend/.env.example backend/.env
 ```
 
 2. Start Postgres and the API:
@@ -33,26 +35,32 @@ cp .env.example .env
 docker compose -f infra/docker/compose.local.yml up --build
 ```
 
-3. Open the Apple workspace:
+3. Check backend health:
+
+```bash
+curl -i http://localhost:3007/api/v1/health
+```
+
+4. Open the Apple workspace:
 
 ```bash
 open apps/apple/Velody.xcworkspace
 ```
 
-## Current Scope
+The local backend runs on `http://localhost:3007`.
 
-This phase intentionally provides:
+## Current Implemented Areas
 
-- backend contracts and scaffolding
-- database schema and initial migration
-- local filesystem storage readiness checks
-- placeholder Apple apps and shared packages
+- upload ingest and transfer flow
+- Apple playback experience
+- iPhone library sync and offline playback resilience
+- account and device-linking foundation
+- billing and entitlement foundation work
 
-This phase intentionally does not yet implement:
+## Not Yet Exposed Public Areas
 
-- folder watching
-- upload chunk transfer
-- background downloads
-- audio playback
-
-The local backend now defaults to `http://localhost:3007`.
+- public login and signup APIs
+- public account authentication APIs
+- Paddle checkout
+- Paddle webhooks
+- public billing APIs

backend/.env.example

@@ -5,3 +5,12 @@ STORAGE_ROOT=../runtime/storage
 PUBLIC_BASE_URL=http://localhost:3007
 DEVICE_BOOTSTRAP_SECRET=replace-me
 MAX_UPLOAD_SIZE_BYTES=524288000
+
+ACCOUNT_ACCESS_TOKEN_SECRET=replace-with-account-access-token-secret
+ACCOUNT_ACCESS_TOKEN_TTL_SECONDS=900
+
+PADDLE_ENVIRONMENT=sandbox
+PADDLE_API_KEY=replace-with-paddle-api-key
+PADDLE_WEBHOOK_SECRET=replace-with-paddle-webhook-secret
+PADDLE_PRO_MONTHLY_PRICE_ID=replace-with-paddle-monthly-price-id
+PADDLE_PRO_YEARLY_PRICE_ID=replace-with-paddle-yearly-price-id

backend/Dockerfile

@@ -20,9 +20,5 @@ COPY --from=builder /app/backend/node_modules ./node_modules
 COPY --from=builder /app/backend/dist ./dist
 COPY --from=builder /app/backend/prisma ./prisma
 
-WORKDIR /app
-COPY runtime ./runtime
-
-WORKDIR /app/backend
-EXPOSE 3000
+EXPOSE 3007
 CMD ["sh", "-c", "npx prisma migrate deploy && node dist/main.js"]

backend/openapi/velody.openapi.json

@@ -1,6 +1,77 @@
 {
   "openapi": "3.0.0",
   "paths": {
+    "/api/v1/billing/webhook": {
+      "post": {
+        "description": "Stores and deduplicates raw Paddle webhook events. Signature verification is not implemented in milestone 12.2.1 and is a known limitation until milestone 12.2.2.",
+        "operationId": "BillingController_ingestWebhook_v1",
+        "parameters": [],
+        "responses": {
+          "200": {
+            "description": "",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/BillingWebhookResponseDto"
+                }
+              }
+            }
+          },
+          "400": {
+            "description": "Invalid webhook payload"
+          }
+        },
+        "summary": "Receive Paddle webhook events",
+        "tags": [
+          "billing"
+        ]
+      }
+    },
+    "/api/v1/billing/checkout": {
+      "post": {
+        "operationId": "BillingController_createCheckout_v1",
+        "parameters": [],
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/CreateBillingCheckoutRequestDto"
+              }
+            }
+          }
+        },
+        "responses": {
+          "201": {
+            "description": "",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/CreateBillingCheckoutResponseDto"
+                }
+              }
+            }
+          },
+          "400": {
+            "description": "Invalid checkout request"
+          },
+          "401": {
+            "description": "Account authentication required"
+          },
+          "502": {
+            "description": "Billing provider customer or checkout creation failed"
+          }
+        },
+        "security": [
+          {
+            "bearer": []
+          }
+        ],
+        "tags": [
+          "billing"
+        ]
+      }
+    },
     "/api/v1/me": {
       "get": {
         "operationId": "AccountController_getMe_v1",
@@ -506,10 +577,57 @@
         "scheme": "bearer",
         "bearerFormat": "Bearer",
         "type": "http",
-        "description": "Device access token"
+        "description": "Device access token or account access token"
       }
     },
     "schemas": {
+      "BillingWebhookResponseDto": {
+        "type": "object",
+        "properties": {
+          "ok": {
+            "type": "boolean",
+            "example": true
+          }
+        },
+        "required": [
+          "ok"
+        ]
+      },
+      "CreateBillingCheckoutRequestDto": {
+        "type": "object",
+        "properties": {
+          "plan": {
+            "type": "string",
+            "enum": [
+              "PRO_MONTHLY",
+              "PRO_YEARLY"
+            ]
+          }
+        },
+        "required": [
+          "plan"
+        ]
+      },
+      "CreateBillingCheckoutResponseDto": {
+        "type": "object",
+        "properties": {
+          "plan": {
+            "type": "string",
+            "enum": [
+              "PRO_MONTHLY",
+              "PRO_YEARLY"
+            ]
+          },
+          "checkoutUrl": {
+            "type": "string",
+            "example": "https://sandbox-checkout.paddle.com/checkout/txn_123"
+          }
+        },
+        "required": [
+          "plan",
+          "checkoutUrl"
+        ]
+      },
       "CurrentAccountResponseDto": {
         "type": "object",
         "properties": {

backend/prisma/migrations/20260708130000_milestone121_checkout_customer_foundation_correction/migration.sql

@@ -0,0 +1,45 @@
+CREATE TABLE "billing_customers" (
+  "id" UUID NOT NULL,
+  "user_id" UUID NOT NULL,
+  "provider" "BillingProvider" NOT NULL,
+  "provider_customer_id" TEXT NOT NULL,
+  "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  "updated_at" TIMESTAMP(3) NOT NULL,
+  CONSTRAINT "billing_customers_pkey" PRIMARY KEY ("id")
+);
+
+CREATE UNIQUE INDEX "billing_customers_user_id_key"
+ON "billing_customers"("user_id");
+
+CREATE UNIQUE INDEX "billing_customers_provider_customer_id_key"
+ON "billing_customers"("provider_customer_id");
+
+ALTER TABLE "billing_customers"
+ADD CONSTRAINT "billing_customers_user_id_fkey"
+FOREIGN KEY ("user_id") REFERENCES "users"("id")
+ON DELETE CASCADE
+ON UPDATE CASCADE;
+
+INSERT INTO "billing_customers" (
+  "id",
+  "user_id",
+  "provider",
+  "provider_customer_id",
+  "created_at",
+  "updated_at"
+)
+SELECT
+  gen_random_uuid(),
+  "user_id",
+  "provider",
+  "provider_customer_id",
+  NOW(),
+  NOW()
+FROM "user_subscriptions"
+WHERE "provider_customer_id" IS NOT NULL
+ON CONFLICT ("user_id") DO NOTHING;
+
+DROP INDEX "user_subscriptions_provider_customer_id_key";
+
+ALTER TABLE "user_subscriptions"
+DROP COLUMN "provider_customer_id";

backend/prisma/migrations/20260709110000_milestone1221_paddle_webhook_ingestion_foundation/migration.sql

@@ -0,0 +1,23 @@
+-- CreateEnum
+CREATE TYPE "BillingWebhookStatus" AS ENUM ('RECEIVED', 'DUPLICATE', 'FAILED');
+
+-- CreateTable
+CREATE TABLE "billing_webhook_events" (
+    "id" UUID NOT NULL,
+    "provider" "BillingProvider" NOT NULL,
+    "provider_event_id" TEXT NOT NULL,
+    "event_type" TEXT NOT NULL,
+    "payload" JSONB NOT NULL,
+    "received_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "processed_at" TIMESTAMP(3),
+    "status" "BillingWebhookStatus" NOT NULL,
+    "error_message" TEXT,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "billing_webhook_events_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "billing_webhook_events_provider_provider_event_id_key"
+ON "billing_webhook_events"("provider", "provider_event_id");

backend/prisma/schema.prisma

@@ -34,6 +34,7 @@ model User {
   incomingTransfers       OwnershipTransfer[]      @relation("OwnershipTransferToUser")
   initiatedTransfers      OwnershipTransfer[]      @relation("OwnershipTransferRequestedByUser")
   accountSessions         AccountSession[]
+  billingCustomer         BillingCustomer?
   subscription            UserSubscription?
   entitlements            UserEntitlement[]
 
@@ -304,11 +305,22 @@ model AccountSession {
   @@map("account_sessions")
 }
 
+model BillingCustomer {
+  id                 String          @id @default(uuid()) @db.Uuid
+  userId             String          @unique @map("user_id") @db.Uuid
+  provider           BillingProvider
+  providerCustomerId String          @unique @map("provider_customer_id")
+  createdAt          DateTime        @default(now()) @map("created_at")
+  updatedAt          DateTime        @updatedAt @map("updated_at")
+  user               User            @relation(fields: [userId], references: [id], onDelete: Cascade, onUpdate: Cascade)
+
+  @@map("billing_customers")
+}
+
 model UserSubscription {
   id                     String             @id @default(uuid()) @db.Uuid
   userId                 String             @unique @map("user_id") @db.Uuid
   provider               BillingProvider
-  providerCustomerId     String?            @unique @map("provider_customer_id")
   providerSubscriptionId String?            @unique @map("provider_subscription_id")
   plan                   SubscriptionPlan
   status                 SubscriptionStatus
@@ -337,6 +349,23 @@ model UserEntitlement {
   @@map("user_entitlements")
 }
 
+model BillingWebhookEvent {
+  id              String               @id @default(uuid()) @db.Uuid
+  provider        BillingProvider
+  providerEventId String               @map("provider_event_id")
+  eventType       String               @map("event_type")
+  payload         Json
+  receivedAt      DateTime             @default(now()) @map("received_at")
+  processedAt     DateTime?            @map("processed_at")
+  status          BillingWebhookStatus
+  errorMessage    String?              @map("error_message")
+  createdAt       DateTime             @default(now()) @map("created_at")
+  updatedAt       DateTime             @updatedAt @map("updated_at")
+
+  @@unique([provider, providerEventId])
+  @@map("billing_webhook_events")
+}
+
 enum SubscriptionStatus {
   TRIAL
   ACTIVE
@@ -354,6 +383,12 @@ enum BillingProvider {
   PADDLE
 }
 
+enum BillingWebhookStatus {
+  RECEIVED
+  DUPLICATE
+  FAILED
+}
+
 enum UserAccountKind {
   LEGACY_DEFAULT
   GUEST

backend/scripts/generate-openapi.ts

@@ -37,7 +37,7 @@ async function generate(): Promise<void> {
           type: 'http',
           scheme: 'bearer',
           bearerFormat: 'Bearer',
-          description: 'Device access token',
+          description: 'Device access token or account access token',
         },
         'bearer',
       )

backend/src/app.factory.ts

@@ -42,7 +42,7 @@ export async function createApp(): Promise<NestExpressApplication> {
           type: 'http',
           scheme: 'bearer',
           bearerFormat: 'Bearer',
-          description: 'Device access token',
+          description: 'Device access token or account access token',
         },
         'bearer',
       )

backend/src/modules/auth/auth.module.ts

@@ -41,6 +41,6 @@ export class AuthModule implements NestModule {
   configure(consumer: MiddlewareConsumer): void {
     consumer
       .apply(RequestContextMiddleware, ProtectedDeviceAuthMiddleware)
-      .forRoutes('*');
+      .forRoutes('{*path}');
   }
 }

backend/src/modules/billing/billing-provider.interface.ts

@@ -1,20 +1,35 @@
 export const BILLING_PROVIDER = Symbol('BILLING_PROVIDER');
 
-/**
- * Provider-neutral contract for future billing integrations.
- *
- * Arguments and return values intentionally remain opaque until the billing
- * domain models are introduced. This keeps the foundation internal and avoids
- * prematurely exposing a provider-specific API.
- */
+export interface BillingCustomerInput {
+  email: string;
+  name: string;
+  accountId: string;
+}
+
+export interface BillingCustomer {
+  id: string;
+}
+
+export interface BillingCheckoutSessionInput {
+  customerId: string;
+  priceId: string;
+  accountId: string;
+}
+
+export interface BillingCheckoutSession {
+  url: string;
+}
+
 export interface BillingProvider {
-  createCustomer(...args: unknown[]): Promise<unknown>;
+  createCustomer(input: BillingCustomerInput): Promise<BillingCustomer>;
   createSubscription(...args: unknown[]): Promise<unknown>;
   updateSubscription(...args: unknown[]): Promise<unknown>;
   cancelSubscription(...args: unknown[]): Promise<unknown>;
   reactivateSubscription(...args: unknown[]): Promise<unknown>;
   getSubscription(...args: unknown[]): Promise<unknown>;
-  createCheckoutSession(...args: unknown[]): Promise<unknown>;
+  createCheckoutSession(
+    input: BillingCheckoutSessionInput,
+  ): Promise<BillingCheckoutSession>;
   createCustomerPortalSession(...args: unknown[]): Promise<unknown>;
   validateWebhookSignature(...args: unknown[]): Promise<unknown>;
   handleWebhookEvent(...args: unknown[]): Promise<unknown>;

backend/src/modules/billing/billing-provider.service.ts

@@ -1,6 +1,10 @@
 import { Inject, Injectable } from '@nestjs/common';
 import {
   BILLING_PROVIDER,
+  BillingCheckoutSession,
+  BillingCheckoutSessionInput,
+  BillingCustomer,
+  BillingCustomerInput,
   BillingProvider,
 } from './billing-provider.interface';
 
@@ -12,8 +16,8 @@ export class BillingProviderService implements BillingProvider {
     private readonly provider: BillingProvider,
   ) {}
 
-  createCustomer(...args: unknown[]): Promise<unknown> {
-    return this.provider.createCustomer(...args);
+  createCustomer(input: BillingCustomerInput): Promise<BillingCustomer> {
+    return this.provider.createCustomer(input);
   }
 
   createSubscription(...args: unknown[]): Promise<unknown> {
@@ -36,8 +40,10 @@ export class BillingProviderService implements BillingProvider {
     return this.provider.getSubscription(...args);
   }
 
-  createCheckoutSession(...args: unknown[]): Promise<unknown> {
-    return this.provider.createCheckoutSession(...args);
+  createCheckoutSession(
+    input: BillingCheckoutSessionInput,
+  ): Promise<BillingCheckoutSession> {
+    return this.provider.createCheckoutSession(input);
   }
 
   createCustomerPortalSession(...args: unknown[]): Promise<unknown> {

backend/src/modules/billing/billing.controller.ts

@@ -0,0 +1,67 @@
+import {
+  Body,
+  Controller,
+  HttpCode,
+  HttpStatus,
+  Post,
+  UseGuards,
+} from '@nestjs/common';
+import {
+  ApiBadGatewayResponse,
+  ApiBadRequestResponse,
+  ApiBearerAuth,
+  ApiCreatedResponse,
+  ApiOkResponse,
+  ApiOperation,
+  ApiTags,
+  ApiUnauthorizedResponse,
+} from '@nestjs/swagger';
+import { AccountAuthGuard } from '../auth/account-auth.guard';
+import { CurrentAccount } from '../auth/current-account.decorator';
+import type { AccountAuthContext } from '../auth/account-auth-context';
+import {
+  BillingWebhookResponseDto,
+  CreateBillingCheckoutRequestDto,
+  CreateBillingCheckoutResponseDto,
+} from './billing.dto';
+import { BillingService } from './billing.service';
+
+@ApiTags('billing')
+@Controller({
+  path: 'billing',
+  version: '1',
+})
+export class BillingController {
+  constructor(private readonly billingService: BillingService) {}
+
+  @Post('webhook')
+  @ApiOperation({
+    summary: 'Receive Paddle webhook events',
+    description:
+      'Stores and deduplicates raw Paddle webhook events. Signature verification is not implemented in milestone 12.2.1 and is a known limitation until milestone 12.2.2.',
+  })
+  @HttpCode(HttpStatus.OK)
+  @ApiOkResponse({ type: BillingWebhookResponseDto })
+  @ApiBadRequestResponse({ description: 'Invalid webhook payload' })
+  async ingestWebhook(
+    @Body() payload: Record<string, unknown>,
+  ): Promise<BillingWebhookResponseDto> {
+    return this.billingService.ingestPaddleWebhook(payload);
+  }
+
+  @Post('checkout')
+  @UseGuards(AccountAuthGuard)
+  @ApiBearerAuth()
+  @ApiCreatedResponse({ type: CreateBillingCheckoutResponseDto })
+  @ApiUnauthorizedResponse({ description: 'Account authentication required' })
+  @ApiBadRequestResponse({ description: 'Invalid checkout request' })
+  @ApiBadGatewayResponse({
+    description: 'Billing provider customer or checkout creation failed',
+  })
+  async createCheckout(
+    @CurrentAccount() account: AccountAuthContext,
+    @Body() body: CreateBillingCheckoutRequestDto,
+  ): Promise<CreateBillingCheckoutResponseDto> {
+    return this.billingService.createCheckout(account, body.plan);
+  }
+}

backend/src/modules/billing/billing.dto.ts

@@ -0,0 +1,32 @@
+import { ApiProperty } from '@nestjs/swagger';
+import { IsEnum, IsUrl } from 'class-validator';
+
+export enum BillingCheckoutPlan {
+  PRO_MONTHLY = 'PRO_MONTHLY',
+  PRO_YEARLY = 'PRO_YEARLY',
+}
+
+export class CreateBillingCheckoutRequestDto {
+  @ApiProperty({ enum: BillingCheckoutPlan })
+  @IsEnum(BillingCheckoutPlan)
+  plan!: BillingCheckoutPlan;
+}
+
+export class CreateBillingCheckoutResponseDto {
+  @ApiProperty({ enum: BillingCheckoutPlan })
+  plan!: BillingCheckoutPlan;
+
+  @ApiProperty({
+    example: 'https://sandbox-checkout.paddle.com/checkout/txn_123',
+  })
+  @IsUrl({
+    require_tld: false,
+    require_protocol: true,
+  })
+  checkoutUrl!: string;
+}
+
+export class BillingWebhookResponseDto {
+  @ApiProperty({ example: true })
+  ok!: true;
+}

backend/src/modules/billing/billing.module.spec.ts

@@ -2,8 +2,11 @@ import { readFileSync } from 'node:fs';
 import { join } from 'node:path';
 import { NotImplementedException } from '@nestjs/common';
 import { Test, TestingModule } from '@nestjs/testing';
+import { AppConfigService } from '../config/config.service';
 import {
   BILLING_PROVIDER,
+  BillingCheckoutSessionInput,
+  BillingCustomerInput,
   BillingProvider,
 } from './billing-provider.interface';
 import { BillingProviderService } from './billing-provider.service';
@@ -11,13 +14,11 @@ import { BillingModule } from './billing.module';
 import { PaddleBillingProvider } from './paddle-billing.provider';
 
 const PROVIDER_METHODS = [
-  'createCustomer',
   'createSubscription',
   'updateSubscription',
   'cancelSubscription',
   'reactivateSubscription',
   'getSubscription',
-  'createCheckoutSession',
   'createCustomerPortalSession',
   'validateWebhookSignature',
   'handleWebhookEvent',
@@ -31,7 +32,13 @@ describe('BillingModule', () => {
   beforeAll(async () => {
     moduleRef = await Test.createTestingModule({
       imports: [BillingModule],
-    }).compile();
+    })
+      .overrideProvider(AppConfigService)
+      .useValue({
+        getPaddleEnvironment: jest.fn().mockReturnValue('sandbox'),
+        getPaddleApiKey: jest.fn().mockReturnValue('paddle-key'),
+      })
+      .compile();
     provider = moduleRef.get(PaddleBillingProvider);
     service = moduleRef.get(BillingProviderService);
   });
@@ -61,6 +68,68 @@ describe('BillingModule', () => {
     expect(selectedProvider).toBe(provider);
   });
 
+  it('creates customers through the configured Paddle API', async () => {
+    const input: BillingCustomerInput = {
+      email: 'owner@example.com',
+      name: 'Velody Owner',
+      accountId: 'user-1',
+    };
+    const fetchRequest = jest
+      .spyOn(globalThis, 'fetch')
+      .mockResolvedValue({
+        ok: true,
+        json: async () => ({
+          data: {
+            id: 'ctm_01hv6y1jedq4p1n0yqn5ba3ky4',
+          },
+        }),
+      } as Response);
+
+    await expect(service.createCustomer(input)).resolves.toEqual({
+      id: 'ctm_01hv6y1jedq4p1n0yqn5ba3ky4',
+    });
+    expect(fetchRequest).toHaveBeenCalledWith(
+      'https://sandbox-api.paddle.com/customers',
+      expect.objectContaining({
+        method: 'POST',
+      }),
+    );
+
+    fetchRequest.mockRestore();
+  });
+
+  it('creates checkout sessions through the configured Paddle API', async () => {
+    const input: BillingCheckoutSessionInput = {
+      customerId: 'ctm_01hv6y1jedq4p1n0yqn5ba3ky4',
+      priceId: 'pri_01gsz96z29d88jrmsf2ztbfgjg',
+      accountId: 'user-1',
+    };
+    const fetchRequest = jest
+      .spyOn(globalThis, 'fetch')
+      .mockResolvedValue({
+        ok: true,
+        json: async () => ({
+          data: {
+            checkout: {
+              url: 'https://checkout.paddle.test/session',
+            },
+          },
+        }),
+      } as Response);
+
+    await expect(service.createCheckoutSession(input)).resolves.toEqual({
+      url: 'https://checkout.paddle.test/session',
+    });
+    expect(fetchRequest).toHaveBeenCalledWith(
+      'https://sandbox-api.paddle.com/transactions',
+      expect.objectContaining({
+        method: 'POST',
+      }),
+    );
+
+    fetchRequest.mockRestore();
+  });
+
   it.each(PROVIDER_METHODS)(
     'throws NotImplementedException from provider method %s',
     async (method) => {
@@ -113,7 +182,7 @@ describe('BillingModule', () => {
     );
   });
 
-  it('performs no HTTP requests while all operations reject', async () => {
+  it('performs no HTTP requests while unimplemented operations reject', async () => {
     const http = jest.requireActual<typeof import('node:http')>('node:http');
     const https = jest.requireActual<typeof import('node:https')>('node:https');
     const httpRequest = jest.spyOn(http, 'request');

backend/src/modules/billing/billing.module.ts

@@ -1,9 +1,16 @@
 import { Module } from '@nestjs/common';
+import { PrismaModule } from '../../infrastructure/database/prisma.module';
+import { AuthModule } from '../auth/auth.module';
+import { AppConfigModule } from '../config/config.module';
 import { BILLING_PROVIDER } from './billing-provider.interface';
+import { BillingController } from './billing.controller';
 import { BillingProviderService } from './billing-provider.service';
+import { BillingService } from './billing.service';
 import { PaddleBillingProvider } from './paddle-billing.provider';
 
 @Module({
+  imports: [AppConfigModule, PrismaModule, AuthModule],
+  controllers: [BillingController],
   providers: [
     PaddleBillingProvider,
     {
@@ -11,7 +18,8 @@ import { PaddleBillingProvider } from './paddle-billing.provider';
       useExisting: PaddleBillingProvider,
     },
     BillingProviderService,
+    BillingService,
   ],
-  exports: [BillingProviderService],
+  exports: [BillingProviderService, BillingService],
 })
 export class BillingModule {}

backend/src/modules/billing/billing.service.spec.ts

@@ -0,0 +1,430 @@
+import {
+  BadGatewayException,
+  BadRequestException,
+  NotFoundException,
+} from '@nestjs/common';
+import {
+  BillingProvider as BillingProviderKind,
+  BillingWebhookStatus,
+  UserAccountStatus,
+} from '@prisma/client';
+import { BillingCheckoutPlan } from './billing.dto';
+import { BillingProviderService } from './billing-provider.service';
+import { BillingService } from './billing.service';
+
+describe('BillingService', () => {
+  function buildService() {
+    const userFindUnique = jest.fn();
+    const billingCustomerCreate = jest.fn();
+    const billingWebhookEventFindUnique = jest.fn();
+    const billingWebhookEventCreate = jest.fn();
+    const billingWebhookEventUpdate = jest.fn();
+    const userSubscriptionUpsert = jest.fn();
+    const userEntitlementCreateMany = jest.fn();
+    const prismaService = {
+      user: {
+        findUnique: userFindUnique,
+      },
+      billingCustomer: {
+        create: billingCustomerCreate,
+      },
+      billingWebhookEvent: {
+        findUnique: billingWebhookEventFindUnique,
+        create: billingWebhookEventCreate,
+        update: billingWebhookEventUpdate,
+      },
+      userSubscription: {
+        upsert: userSubscriptionUpsert,
+      },
+      userEntitlement: {
+        createMany: userEntitlementCreateMany,
+      },
+    } as any;
+    const billingProviderService = {
+      createCustomer: jest.fn(),
+      createCheckoutSession: jest.fn(),
+    } as unknown as BillingProviderService;
+    const configService = {
+      getPaddleProMonthlyPriceId: jest.fn().mockReturnValue('pri_monthly'),
+      getPaddleProYearlyPriceId: jest.fn().mockReturnValue('pri_yearly'),
+    } as any;
+
+    return {
+      service: new BillingService(
+        prismaService,
+        billingProviderService,
+        configService,
+      ),
+      prismaService,
+      billingProviderService: billingProviderService as any,
+      configService,
+      mocks: {
+        billingCustomerCreate,
+        billingWebhookEventFindUnique,
+        billingWebhookEventCreate,
+        billingWebhookEventUpdate,
+        userSubscriptionUpsert,
+        userEntitlementCreateMany,
+      },
+    };
+  }
+
+  it('creates a checkout with a new Paddle customer when none exists', async () => {
+    const { service, prismaService, billingProviderService, mocks } =
+      buildService();
+
+    prismaService.user.findUnique.mockResolvedValue({
+      id: 'user-1',
+      displayName: 'Velody Owner',
+      accountStatus: UserAccountStatus.ACTIVE,
+      oauthIdentities: [{ email: 'owner@example.com' }],
+      billingCustomer: null,
+    });
+    billingProviderService.createCustomer.mockResolvedValue({ id: 'ctm_123' });
+    mocks.billingCustomerCreate.mockResolvedValue({
+      providerCustomerId: 'ctm_123',
+    });
+    billingProviderService.createCheckoutSession.mockResolvedValue({
+      url: 'https://checkout.example/session',
+    });
+
+    await expect(
+      service.createCheckout(
+        {
+          userId: 'user-1',
+          sessionId: 'session-1',
+          accessTokenVersion: 1,
+        },
+        BillingCheckoutPlan.PRO_MONTHLY,
+      ),
+    ).resolves.toEqual({
+      plan: BillingCheckoutPlan.PRO_MONTHLY,
+      checkoutUrl: 'https://checkout.example/session',
+    });
+
+    expect(billingProviderService.createCustomer).toHaveBeenCalledWith({
+      email: 'owner@example.com',
+      name: 'Velody Owner',
+      accountId: 'user-1',
+    });
+    expect(mocks.billingCustomerCreate).toHaveBeenCalledWith({
+      data: {
+        userId: 'user-1',
+        provider: BillingProviderKind.PADDLE,
+        providerCustomerId: 'ctm_123',
+      },
+      select: {
+        providerCustomerId: true,
+      },
+    });
+    expect(mocks.userSubscriptionUpsert).not.toHaveBeenCalled();
+    expect(mocks.userEntitlementCreateMany).not.toHaveBeenCalled();
+    expect(billingProviderService.createCheckoutSession).toHaveBeenCalledWith({
+      customerId: 'ctm_123',
+      priceId: 'pri_monthly',
+      accountId: 'user-1',
+    });
+  });
+
+  it('reuses an existing Paddle customer', async () => {
+    const { service, prismaService, billingProviderService, mocks } =
+      buildService();
+
+    prismaService.user.findUnique.mockResolvedValue({
+      id: 'user-1',
+      displayName: 'Velody Owner',
+      accountStatus: UserAccountStatus.ACTIVE,
+      oauthIdentities: [{ email: 'owner@example.com' }],
+      billingCustomer: {
+        id: 'billing-customer-1',
+        providerCustomerId: 'ctm_existing',
+      },
+    });
+    billingProviderService.createCheckoutSession.mockResolvedValue({
+      url: 'https://checkout.example/existing',
+    });
+
+    await service.createCheckout(
+      {
+        userId: 'user-1',
+        sessionId: 'session-1',
+        accessTokenVersion: 1,
+      },
+      BillingCheckoutPlan.PRO_YEARLY,
+    );
+
+    expect(billingProviderService.createCustomer).not.toHaveBeenCalled();
+    expect(mocks.billingCustomerCreate).not.toHaveBeenCalled();
+    expect(mocks.userSubscriptionUpsert).not.toHaveBeenCalled();
+    expect(mocks.userEntitlementCreateMany).not.toHaveBeenCalled();
+    expect(billingProviderService.createCheckoutSession).toHaveBeenCalledWith({
+      customerId: 'ctm_existing',
+      priceId: 'pri_yearly',
+      accountId: 'user-1',
+    });
+  });
+
+  it('rejects checkout when the account has no stored email', async () => {
+    const { service, prismaService, mocks } = buildService();
+
+    prismaService.user.findUnique.mockResolvedValue({
+      id: 'user-1',
+      displayName: 'Velody Owner',
+      accountStatus: UserAccountStatus.ACTIVE,
+      oauthIdentities: [],
+      billingCustomer: null,
+    });
+
+    await expect(
+      service.createCheckout(
+        {
+          userId: 'user-1',
+          sessionId: 'session-1',
+          accessTokenVersion: 1,
+        },
+        BillingCheckoutPlan.PRO_MONTHLY,
+      ),
+    ).rejects.toThrow(
+      new BadRequestException('Account email is required for checkout'),
+    );
+
+    expect(mocks.userSubscriptionUpsert).not.toHaveBeenCalled();
+    expect(mocks.userEntitlementCreateMany).not.toHaveBeenCalled();
+  });
+
+  it('maps Paddle customer creation failures to a gateway error', async () => {
+    const { service, prismaService, billingProviderService, mocks } =
+      buildService();
+
+    prismaService.user.findUnique.mockResolvedValue({
+      id: 'user-1',
+      displayName: 'Velody Owner',
+      accountStatus: UserAccountStatus.ACTIVE,
+      oauthIdentities: [{ email: 'owner@example.com' }],
+      billingCustomer: null,
+    });
+    billingProviderService.createCustomer.mockRejectedValue(new Error('boom'));
+
+    await expect(
+      service.createCheckout(
+        {
+          userId: 'user-1',
+          sessionId: 'session-1',
+          accessTokenVersion: 1,
+        },
+        BillingCheckoutPlan.PRO_MONTHLY,
+      ),
+    ).rejects.toThrow(
+      new BadGatewayException('Unable to create billing customer'),
+    );
+
+    expect(mocks.userSubscriptionUpsert).not.toHaveBeenCalled();
+    expect(mocks.userEntitlementCreateMany).not.toHaveBeenCalled();
+  });
+
+  it('maps checkout session failures to a gateway error', async () => {
+    const { service, prismaService, billingProviderService, mocks } =
+      buildService();
+
+    prismaService.user.findUnique.mockResolvedValue({
+      id: 'user-1',
+      displayName: 'Velody Owner',
+      accountStatus: UserAccountStatus.ACTIVE,
+      oauthIdentities: [{ email: 'owner@example.com' }],
+      billingCustomer: {
+        id: 'billing-customer-1',
+        providerCustomerId: 'ctm_existing',
+      },
+    });
+    billingProviderService.createCheckoutSession.mockRejectedValue(
+      new Error('boom'),
+    );
+
+    await expect(
+      service.createCheckout(
+        {
+          userId: 'user-1',
+          sessionId: 'session-1',
+          accessTokenVersion: 1,
+        },
+        BillingCheckoutPlan.PRO_MONTHLY,
+      ),
+    ).rejects.toThrow(
+      new BadGatewayException('Unable to create checkout session'),
+    );
+
+    expect(mocks.userSubscriptionUpsert).not.toHaveBeenCalled();
+    expect(mocks.userEntitlementCreateMany).not.toHaveBeenCalled();
+  });
+
+  it('rejects missing or inactive accounts', async () => {
+    const { service, prismaService, mocks } = buildService();
+
+    prismaService.user.findUnique.mockResolvedValueOnce(null);
+
+    await expect(
+      service.createCheckout(
+        {
+          userId: 'missing-user',
+          sessionId: 'session-1',
+          accessTokenVersion: 1,
+        },
+        BillingCheckoutPlan.PRO_MONTHLY,
+      ),
+    ).rejects.toThrow(new NotFoundException('Account not found'));
+
+    prismaService.user.findUnique.mockResolvedValueOnce({
+      id: 'user-1',
+      displayName: 'Velody Owner',
+      accountStatus: UserAccountStatus.LOCKED,
+      oauthIdentities: [{ email: 'owner@example.com' }],
+      billingCustomer: null,
+    });
+
+    await expect(
+      service.createCheckout(
+        {
+          userId: 'user-1',
+          sessionId: 'session-1',
+          accessTokenVersion: 1,
+        },
+        BillingCheckoutPlan.PRO_MONTHLY,
+      ),
+    ).rejects.toThrow(new BadRequestException('Account is not active'));
+
+    expect(mocks.userSubscriptionUpsert).not.toHaveBeenCalled();
+    expect(mocks.userEntitlementCreateMany).not.toHaveBeenCalled();
+  });
+
+  it('stores a valid webhook event', async () => {
+    const { service, mocks } = buildService();
+    const payload = {
+      event_id: 'evt_123',
+      event_type: 'transaction.completed',
+      data: {
+        id: 'txn_123',
+      },
+    };
+
+    mocks.billingWebhookEventFindUnique.mockResolvedValue(null);
+
+    await expect(service.ingestPaddleWebhook(payload)).resolves.toEqual({
+      ok: true,
+    });
+
+    expect(mocks.billingWebhookEventCreate).toHaveBeenCalledWith({
+      data: {
+        provider: BillingProviderKind.PADDLE,
+        providerEventId: 'evt_123',
+        eventType: 'transaction.completed',
+        payload,
+        status: BillingWebhookStatus.RECEIVED,
+      },
+    });
+  });
+
+  it('does not store a duplicate webhook twice', async () => {
+    const { service, mocks } = buildService();
+
+    mocks.billingWebhookEventFindUnique.mockResolvedValue({
+      id: 'webhook-1',
+    });
+
+    await expect(
+      service.ingestPaddleWebhook({
+        event_id: 'evt_duplicate',
+        event_type: 'subscription.updated',
+      }),
+    ).resolves.toEqual({
+      ok: true,
+    });
+
+    expect(mocks.billingWebhookEventCreate).not.toHaveBeenCalled();
+    expect(mocks.billingWebhookEventUpdate).toHaveBeenCalledWith({
+      where: {
+        id: 'webhook-1',
+      },
+      data: {
+        status: BillingWebhookStatus.DUPLICATE,
+      },
+    });
+  });
+
+  it('rejects missing webhook event ID', async () => {
+    const { service, mocks } = buildService();
+
+    await expect(
+      service.ingestPaddleWebhook({
+        event_type: 'transaction.completed',
+      }),
+    ).rejects.toThrow(
+      new BadRequestException('Webhook payload event_id is required'),
+    );
+
+    expect(mocks.billingWebhookEventCreate).not.toHaveBeenCalled();
+  });
+
+  it('rejects missing webhook event type', async () => {
+    const { service, mocks } = buildService();
+
+    await expect(
+      service.ingestPaddleWebhook({
+        event_id: 'evt_missing_type',
+      }),
+    ).rejects.toThrow(
+      new BadRequestException('Webhook payload event_type is required'),
+    );
+
+    expect(mocks.billingWebhookEventCreate).not.toHaveBeenCalled();
+  });
+
+  it('stores unknown webhook event types without processing', async () => {
+    const { service, mocks } = buildService();
+    const payload = {
+      event_id: 'evt_unknown',
+      event_type: 'reporting.exported',
+    };
+
+    mocks.billingWebhookEventFindUnique.mockResolvedValue(null);
+
+    await expect(service.ingestPaddleWebhook(payload)).resolves.toEqual({
+      ok: true,
+    });
+
+    expect(mocks.billingWebhookEventCreate).toHaveBeenCalledWith({
+      data: {
+        provider: BillingProviderKind.PADDLE,
+        providerEventId: 'evt_unknown',
+        eventType: 'reporting.exported',
+        payload,
+        status: BillingWebhookStatus.RECEIVED,
+      },
+    });
+  });
+
+  it('stores the webhook payload as JSON and does not write subscriptions or entitlements', async () => {
+    const { service, mocks } = buildService();
+    const payload = {
+      event_id: 'evt_payload',
+      event_type: 'subscription.created',
+      data: {
+        items: [{ price_id: 'pri_123' }],
+        custom_data: {
+          velodyAccountId: 'user-1',
+        },
+      },
+    };
+
+    mocks.billingWebhookEventFindUnique.mockResolvedValue(null);
+
+    await service.ingestPaddleWebhook(payload);
+
+    expect(mocks.billingWebhookEventCreate).toHaveBeenCalledWith({
+      data: expect.objectContaining({
+        payload,
+      }),
+    });
+    expect(mocks.userSubscriptionUpsert).not.toHaveBeenCalled();
+    expect(mocks.userEntitlementCreateMany).not.toHaveBeenCalled();
+  });
+});

backend/src/modules/billing/billing.service.ts

@@ -0,0 +1,260 @@
+import {
+  BadGatewayException,
+  BadRequestException,
+  Injectable,
+  NotFoundException,
+} from '@nestjs/common';
+import {
+  BillingProvider as BillingProviderKind,
+  BillingWebhookStatus,
+  Prisma,
+  UserAccountStatus,
+} from '@prisma/client';
+import { PrismaService } from '../../infrastructure/database/prisma.service';
+import type { AccountAuthContext } from '../auth/account-auth-context';
+import { AppConfigService } from '../config/config.service';
+import { BillingProviderService } from './billing-provider.service';
+import {
+  BillingWebhookResponseDto,
+  BillingCheckoutPlan,
+  CreateBillingCheckoutResponseDto,
+} from './billing.dto';
+
+interface CheckoutAccountRecord {
+  id: string;
+  displayName: string;
+  accountStatus: UserAccountStatus;
+  oauthIdentities: Array<{
+    email: string | null;
+  }>;
+  billingCustomer: {
+    id: string;
+    providerCustomerId: string | null;
+  } | null;
+}
+
+interface PaddleWebhookPayload {
+  event_id?: unknown;
+  event_type?: unknown;
+  [key: string]: unknown;
+}
+
+const PADDLE_INGESTION_EVENT_TYPES = new Set([
+  'transaction.completed',
+  'subscription.created',
+  'subscription.updated',
+  'subscription.canceled',
+]);
+
+@Injectable()
+export class BillingService {
+  constructor(
+    private readonly prismaService: PrismaService,
+    private readonly billingProviderService: BillingProviderService,
+    private readonly configService: AppConfigService,
+  ) {}
+
+  async createCheckout(
+    account: AccountAuthContext,
+    request: BillingCheckoutPlan,
+  ): Promise<CreateBillingCheckoutResponseDto> {
+    const checkoutAccount = await this.getCheckoutAccount(account.userId);
+    const email = this.getCheckoutEmail(checkoutAccount);
+    const providerCustomerId =
+      checkoutAccount.billingCustomer?.providerCustomerId ??
+      (await this.createAndPersistCustomer(checkoutAccount.id, {
+        email,
+        name: checkoutAccount.displayName,
+      }));
+
+    try {
+      const session = await this.billingProviderService.createCheckoutSession({
+        customerId: providerCustomerId,
+        priceId: this.resolvePriceId(request),
+        accountId: checkoutAccount.id,
+      });
+
+      return {
+        plan: request,
+        checkoutUrl: session.url,
+      };
+    } catch {
+      throw new BadGatewayException('Unable to create checkout session');
+    }
+  }
+
+  async ingestPaddleWebhook(
+    payload: Record<string, unknown>,
+  ): Promise<BillingWebhookResponseDto> {
+    const { eventId, eventType } = this.extractWebhookMetadata(payload);
+
+    const existingEvent =
+      await this.prismaService.billingWebhookEvent.findUnique({
+        where: {
+          provider_providerEventId: {
+            provider: BillingProviderKind.PADDLE,
+            providerEventId: eventId,
+          },
+        },
+        select: {
+          id: true,
+        },
+      });
+
+    if (existingEvent) {
+      await this.prismaService.billingWebhookEvent.update({
+        where: {
+          id: existingEvent.id,
+        },
+        data: {
+          status: BillingWebhookStatus.DUPLICATE,
+        },
+      });
+
+      return { ok: true };
+    }
+
+    await this.prismaService.billingWebhookEvent.create({
+      data: {
+        provider: BillingProviderKind.PADDLE,
+        providerEventId: eventId,
+        eventType,
+        payload: payload as Prisma.InputJsonValue,
+        status: BillingWebhookStatus.RECEIVED,
+      },
+    });
+
+    if (!PADDLE_INGESTION_EVENT_TYPES.has(eventType)) {
+      return { ok: true };
+    }
+
+    return { ok: true };
+  }
+
+  private async getCheckoutAccount(
+    userId: string,
+  ): Promise<CheckoutAccountRecord> {
+    const account = await this.prismaService.user.findUnique({
+      where: {
+        id: userId,
+      },
+      select: {
+        id: true,
+        displayName: true,
+        accountStatus: true,
+        oauthIdentities: {
+          select: {
+            email: true,
+          },
+          orderBy: {
+            createdAt: 'asc',
+          },
+        },
+        billingCustomer: {
+          select: {
+            id: true,
+            providerCustomerId: true,
+          },
+        },
+      },
+    });
+
+    if (!account) {
+      throw new NotFoundException('Account not found');
+    }
+
+    if (account.accountStatus !== UserAccountStatus.ACTIVE) {
+      throw new BadRequestException('Account is not active');
+    }
+
+    return account;
+  }
+
+  private getCheckoutEmail(account: CheckoutAccountRecord): string {
+    const email = account.oauthIdentities.find((identity) => identity.email)
+      ?.email;
+
+    if (!email) {
+      throw new BadRequestException('Account email is required for checkout');
+    }
+
+    return email;
+  }
+
+  private async createAndPersistCustomer(
+    userId: string,
+    customer: {
+      email: string;
+      name: string;
+    },
+  ): Promise<string> {
+    try {
+      const createdCustomer = await this.billingProviderService.createCustomer({
+        email: customer.email,
+        name: customer.name,
+        accountId: userId,
+      });
+
+      const billingCustomer = await this.prismaService.billingCustomer.create({
+        data: {
+          userId,
+          provider: BillingProviderKind.PADDLE,
+          providerCustomerId: createdCustomer.id,
+        },
+        select: {
+          providerCustomerId: true,
+        },
+      });
+
+      if (!billingCustomer.providerCustomerId) {
+        throw new BadGatewayException('Unable to persist billing customer');
+      }
+
+      return billingCustomer.providerCustomerId;
+    } catch (error) {
+      if (error instanceof BadRequestException) {
+        throw error;
+      }
+
+      if (error instanceof BadGatewayException) {
+        throw error;
+      }
+
+      throw new BadGatewayException('Unable to create billing customer');
+    }
+  }
+
+  private resolvePriceId(plan: BillingCheckoutPlan): string {
+    switch (plan) {
+      case BillingCheckoutPlan.PRO_MONTHLY:
+        return this.configService.getPaddleProMonthlyPriceId();
+      case BillingCheckoutPlan.PRO_YEARLY:
+        return this.configService.getPaddleProYearlyPriceId();
+    }
+  }
+
+  private extractWebhookMetadata(payload: PaddleWebhookPayload): {
+    eventId: string;
+    eventType: string;
+  } {
+    if (!payload || typeof payload !== 'object' || Array.isArray(payload)) {
+      throw new BadRequestException('Webhook payload must be a JSON object');
+    }
+
+    const eventId = this.getRequiredString(payload.event_id, 'event_id');
+    const eventType = this.getRequiredString(payload.event_type, 'event_type');
+
+    return {
+      eventId,
+      eventType,
+    };
+  }
+
+  private getRequiredString(value: unknown, field: string): string {
+    if (typeof value !== 'string' || value.trim().length === 0) {
+      throw new BadRequestException(`Webhook payload ${field} is required`);
+    }
+
+    return value;
+  }
+}

backend/src/modules/billing/paddle-billing.provider.ts

@@ -1,10 +1,61 @@
-import { Injectable, NotImplementedException } from '@nestjs/common';
-import { BillingProvider } from './billing-provider.interface';
+import {
+  Injectable,
+  InternalServerErrorException,
+  NotImplementedException,
+} from '@nestjs/common';
+import { AppConfigService } from '../config/config.service';
+import {
+  BillingCheckoutSession,
+  BillingCheckoutSessionInput,
+  BillingCustomer,
+  BillingCustomerInput,
+  BillingProvider,
+} from './billing-provider.interface';
+
+interface PaddleApiEnvelope<T> {
+  data?: T;
+  error?: {
+    type?: string;
+    code?: string;
+    detail?: string;
+  };
+}
+
+interface PaddleCustomerResponse {
+  id: string;
+}
+
+interface PaddleTransactionResponse {
+  checkout?: {
+    url?: string | null;
+  } | null;
+}
 
 @Injectable()
 export class PaddleBillingProvider implements BillingProvider {
-  async createCustomer(..._args: unknown[]): Promise<never> {
-    throw new NotImplementedException();
+  constructor(private readonly configService: AppConfigService) {}
+
+  async createCustomer(input: BillingCustomerInput): Promise<BillingCustomer> {
+    const response = await this.request<PaddleCustomerResponse>('/customers', {
+      method: 'POST',
+      body: JSON.stringify({
+        email: input.email,
+        name: input.name,
+        custom_data: {
+          velodyAccountId: input.accountId,
+        },
+      }),
+    });
+
+    if (!response.data?.id) {
+      throw new InternalServerErrorException(
+        'Billing provider returned an invalid customer response',
+      );
+    }
+
+    return {
+      id: response.data.id,
+    };
   }
 
   async createSubscription(..._args: unknown[]): Promise<never> {
@@ -27,8 +78,40 @@ export class PaddleBillingProvider implements BillingProvider {
     throw new NotImplementedException();
   }
 
-  async createCheckoutSession(..._args: unknown[]): Promise<never> {
-    throw new NotImplementedException();
+  async createCheckoutSession(
+    input: BillingCheckoutSessionInput,
+  ): Promise<BillingCheckoutSession> {
+    const response = await this.request<PaddleTransactionResponse>(
+      '/transactions',
+      {
+        method: 'POST',
+        body: JSON.stringify({
+          items: [
+            {
+              price_id: input.priceId,
+              quantity: 1,
+            },
+          ],
+          customer_id: input.customerId,
+          collection_mode: 'automatic',
+          custom_data: {
+            velodyAccountId: input.accountId,
+          },
+        }),
+      },
+    );
+
+    const checkoutUrl = response.data?.checkout?.url;
+
+    if (!checkoutUrl) {
+      throw new InternalServerErrorException(
+        'Billing provider returned an invalid checkout response',
+      );
+    }
+
+    return {
+      url: checkoutUrl,
+    };
   }
 
   async createCustomerPortalSession(..._args: unknown[]): Promise<never> {
@@ -42,4 +125,40 @@ export class PaddleBillingProvider implements BillingProvider {
   async handleWebhookEvent(..._args: unknown[]): Promise<never> {
     throw new NotImplementedException();
   }
+
+  private async request<T>(
+    path: string,
+    init: RequestInit,
+  ): Promise<PaddleApiEnvelope<T>> {
+    const response = await fetch(`${this.getBaseUrl()}${path}`, {
+      ...init,
+      headers: {
+        Authorization: `Bearer ${this.configService.getPaddleApiKey()}`,
+        'Content-Type': 'application/json',
+        Accept: 'application/json',
+        ...(init.headers ?? {}),
+      },
+    });
+
+    const payload = (await response.json()) as PaddleApiEnvelope<T>;
+
+    if (!response.ok) {
+      const detail =
+        payload.error?.detail ??
+        payload.error?.code ??
+        payload.error?.type ??
+        'Unknown billing provider error';
+      throw new InternalServerErrorException(detail);
+    }
+
+    return payload;
+  }
+
+  private getBaseUrl(): string {
+    if (this.configService.getPaddleEnvironment() === 'production') {
+      return 'https://api.paddle.com';
+    }
+
+    return 'https://sandbox-api.paddle.com';
+  }
 }

backend/src/modules/storage/storage.service.ts

@@ -71,8 +71,6 @@ export class LocalFilesystemStorageService {
       join(this.root, 'temp', 'uploads'),
       join(this.root, 'incoming'),
       join(this.root, 'quarantine'),
-      join(this.root, 'library', 'audio'),
-      join(this.root, 'library', 'artwork'),
     ];
 
     for (const path of paths) {

backend/src/modules/users/billing-customer-migration.spec.ts

@@ -0,0 +1,47 @@
+import { readFile } from 'node:fs/promises';
+import { join } from 'node:path';
+
+describe('billing customer foundation correction migration', () => {
+  let migrationSql: string;
+
+  beforeAll(async () => {
+    migrationSql = await readFile(
+      join(
+        process.cwd(),
+        'prisma/migrations/20260708130000_milestone121_checkout_customer_foundation_correction/migration.sql',
+      ),
+      'utf8',
+    );
+  });
+
+  it('creates the billing_customers table', () => {
+    expect(migrationSql).toContain(`CREATE TABLE "billing_customers"`);
+  });
+
+  it('enforces one billing customer per user and unique provider customer ids', () => {
+    expect(migrationSql).toContain(
+      `CREATE UNIQUE INDEX "billing_customers_user_id_key"`,
+    );
+    expect(migrationSql).toContain(
+      `CREATE UNIQUE INDEX "billing_customers_provider_customer_id_key"`,
+    );
+  });
+
+  it('relates billing customers to users with cascading deletes', () => {
+    expect(migrationSql).toContain(
+      `ADD CONSTRAINT "billing_customers_user_id_fkey"`,
+    );
+    expect(migrationSql).toContain(`REFERENCES "users"("id")`);
+    expect(migrationSql).toContain(`ON DELETE CASCADE`);
+    expect(migrationSql).toContain(`ON UPDATE CASCADE`);
+  });
+
+  it('removes provider customer identity from user_subscriptions', () => {
+    expect(migrationSql).toContain(
+      `DROP INDEX "user_subscriptions_provider_customer_id_key";`,
+    );
+    expect(migrationSql).toContain(
+      `DROP COLUMN "provider_customer_id";`,
+    );
+  });
+});

backend/src/modules/users/subscription-foundation-migration.spec.ts

@@ -31,24 +31,17 @@ describe('subscription foundation migration', () => {
     );
   });
 
-  it('enforces one subscription per user and unique provider identifiers', () => {
+  it('enforces one subscription per user and unique provider subscription identifiers', () => {
     expect(migrationSql).toContain(
       `CREATE UNIQUE INDEX "user_subscriptions_user_id_key"`,
     );
-    expect(migrationSql).toContain(
-      `CREATE UNIQUE INDEX "user_subscriptions_provider_customer_id_key"`,
-    );
     expect(migrationSql).toContain(
       `CREATE UNIQUE INDEX "user_subscriptions_provider_subscription_id_key"`,
     );
   });
 
-  it('keeps provider identifiers nullable', () => {
-    expect(migrationSql).toContain(`"provider_customer_id" TEXT,`);
+  it('keeps the provider subscription identifier nullable', () => {
     expect(migrationSql).toContain(`"provider_subscription_id" TEXT,`);
-    expect(migrationSql).not.toContain(
-      `"provider_customer_id" TEXT NOT NULL`,
-    );
     expect(migrationSql).not.toContain(
       `"provider_subscription_id" TEXT NOT NULL`,
     );

backend/test/e2e/app.e2e-spec.ts

@@ -4,12 +4,17 @@ import { tmpdir } from 'node:os';
 import { dirname, join } from 'node:path';
 import { Readable } from 'node:stream';
 import {
+  BadRequestException,
+  BadGatewayException,
+  ExecutionContext,
   ForbiddenException,
+  HttpStatus,
   NotFoundException,
   UnauthorizedException,
   ValidationPipe,
   VersioningType,
 } from '@nestjs/common';
+import { HTTP_CODE_METADATA } from '@nestjs/common/constants';
 import type { NestExpressApplication } from '@nestjs/platform-express';
 import { Test } from '@nestjs/testing';
 import { API_JSON_BODY_LIMIT } from '../../src/app.factory';
@@ -17,8 +22,15 @@ import { AppModule } from '../../src/app.module';
 import { RequestContextService } from '../../src/infrastructure/request-context/request-context.service';
 import { AssetsController } from '../../src/modules/assets/assets.controller';
 import { AssetDownloadQueryDto } from '../../src/modules/assets/assets.dto';
+import { AccountAuthGuard } from '../../src/modules/auth/account-auth.guard';
+import { AuthenticationRuntimeService } from '../../src/modules/auth/authentication-runtime.service';
 import { DeviceAuthService } from '../../src/modules/auth/device-auth.service';
 import { ArtworkController } from '../../src/modules/artwork/artwork.controller';
+import { BillingController } from '../../src/modules/billing/billing.controller';
+import {
+  BillingCheckoutPlan,
+  CreateBillingCheckoutRequestDto,
+} from '../../src/modules/billing/billing.dto';
 import { AppConfigService } from '../../src/modules/config/config.service';
 import { DevicesController } from '../../src/modules/devices/devices.controller';
 import { HealthController } from '../../src/modules/health/health.controller';
@@ -91,6 +103,11 @@ function createPrismaMock() {
   const artworkAssets = new Map<string, any>();
   const uploadSessions = new Map<string, any>();
   const libraryEvents = new Map<bigint, any>();
+  const billingCustomers = new Map<string, any>();
+  const billingWebhookEvents = new Map<string, any>();
+  const userSubscriptions = new Map<string, any>();
+  const userOAuthIdentities = new Map<string, any>();
+  const userEntitlements = new Map<string, any>();
   let nextLibraryEventId = 1n;
 
   const createUserRecord = (data: Record<string, any>) => {
@@ -143,8 +160,58 @@ function createPrismaMock() {
         return created;
       }),
       findUnique: jest.fn().mockImplementation(async ({ where, select }) => {
+        const attachRelations = (user: Record<string, any> | null) => {
+          if (!user || !select) {
+            return applySelect(user, select);
+          }
+
+          const baseSelect = Object.fromEntries(
+            Object.entries(select).filter(([, value]) => value === true),
+          ) as Record<string, boolean>;
+          const selectedUser = applySelect(user, baseSelect) as Record<
+            string,
+            any
+          >;
+
+          if (select.oauthIdentities) {
+            const identities = [...userOAuthIdentities.values()]
+              .filter((identity) => identity.userId === user.id)
+              .sort(
+                (lhs, rhs) => lhs.createdAt.getTime() - rhs.createdAt.getTime(),
+              )
+              .map((identity) =>
+                applySelect(identity, select.oauthIdentities.select),
+              );
+            selectedUser.oauthIdentities = identities;
+          }
+
+          if (select.billingCustomer) {
+            const billingCustomer =
+              [...billingCustomers.values()].find(
+                (record) => record.userId === user.id,
+              ) ?? null;
+            selectedUser.billingCustomer = applySelect(
+              billingCustomer,
+              select.billingCustomer.select,
+            );
+          }
+
+          if (select.subscription) {
+            const subscription =
+              [...userSubscriptions.values()].find(
+                (record) => record.userId === user.id,
+              ) ?? null;
+            selectedUser.subscription = applySelect(
+              subscription,
+              select.subscription.select,
+            );
+          }
+
+          return selectedUser;
+        };
+
         if (where.id) {
-          return applySelect(users.get(where.id) ?? null, select);
+          return attachRelations(users.get(where.id) ?? null);
         }
 
         if (where.slug) {
@@ -152,7 +219,7 @@ function createPrismaMock() {
             [...users.values()].find((user) => user.slug === where.slug) ??
             null;
 
-          return applySelect(matchingUser, select);
+          return attachRelations(matchingUser);
         }
 
         return null;
@@ -187,6 +254,40 @@ function createPrismaMock() {
         return updated;
       }),
     },
+    userOAuthIdentity: {
+      create: jest.fn().mockImplementation(async ({ data, select }) => {
+        const record = {
+          id: randomUUID(),
+          createdAt: new Date(),
+          updatedAt: new Date(),
+          ...data,
+        };
+        userOAuthIdentities.set(record.id, record);
+        return applySelect(record, select);
+      }),
+      findUnique: jest.fn().mockImplementation(async ({ where, include }) => {
+        const record =
+          [...userOAuthIdentities.values()].find(
+            (identity) =>
+              identity.provider === where.provider_providerSubject?.provider &&
+              identity.providerSubject ===
+                where.provider_providerSubject?.providerSubject,
+          ) ?? null;
+
+        if (!record) {
+          return null;
+        }
+
+        if (include?.user) {
+          return {
+            ...record,
+            user: applySelect(users.get(record.userId) ?? null, include.user.select),
+          };
+        }
+
+        return record;
+      }),
+    },
     device: {
       create: jest.fn().mockImplementation(async ({ data }) => {
         const record = {
@@ -225,6 +326,129 @@ function createPrismaMock() {
         return updated;
       }),
     },
+    billingCustomer: {
+      create: jest.fn().mockImplementation(async ({ data, select }) => {
+        const record = {
+          id: randomUUID(),
+          createdAt: new Date(),
+          updatedAt: new Date(),
+          ...data,
+        };
+        billingCustomers.set(record.id, record);
+        return applySelect(record, select);
+      }),
+      findUnique: jest.fn().mockImplementation(async ({ where, select }) => {
+        const record =
+          [...billingCustomers.values()].find(
+            (billingCustomer) =>
+              billingCustomer.userId === where.userId ||
+              billingCustomer.providerCustomerId === where.providerCustomerId,
+          ) ?? null;
+
+        return applySelect(record, select);
+      }),
+    },
+    billingWebhookEvent: {
+      findUnique: jest.fn().mockImplementation(async ({ where, select }) => {
+        const composite = where.provider_providerEventId;
+        const record =
+          [...billingWebhookEvents.values()].find(
+            (event) =>
+              event.provider === composite?.provider &&
+              event.providerEventId === composite?.providerEventId,
+          ) ?? null;
+
+        return applySelect(record, select);
+      }),
+      create: jest.fn().mockImplementation(async ({ data }) => {
+        const now = new Date();
+        const record = {
+          id: randomUUID(),
+          receivedAt: now,
+          processedAt: null,
+          errorMessage: null,
+          createdAt: now,
+          updatedAt: now,
+          ...data,
+        };
+        billingWebhookEvents.set(record.id, record);
+        return record;
+      }),
+      update: jest.fn().mockImplementation(async ({ where, data }) => {
+        const current = billingWebhookEvents.get(where.id);
+        const updated = {
+          ...current,
+          ...data,
+          updatedAt: new Date(),
+        };
+        billingWebhookEvents.set(where.id, updated);
+        return updated;
+      }),
+    },
+    userSubscription: {
+      findUnique: jest.fn().mockImplementation(async ({ where, select }) => {
+        const record =
+          [...userSubscriptions.values()].find(
+            (subscription) => subscription.userId === where.userId,
+          ) ?? null;
+
+        return applySelect(record, select);
+      }),
+      upsert: jest.fn().mockImplementation(async ({ where, update, create, select }) => {
+        const existing =
+          [...userSubscriptions.values()].find(
+            (subscription) => subscription.userId === where.userId,
+          ) ?? null;
+        const now = new Date();
+        const record = existing
+          ? {
+              ...existing,
+              ...update,
+              updatedAt: now,
+            }
+          : {
+              id: randomUUID(),
+              createdAt: now,
+              updatedAt: now,
+              ...create,
+            };
+
+        userSubscriptions.set(record.id, record);
+        return applySelect(record, select);
+      }),
+    },
+    userEntitlement: {
+      findMany: jest.fn().mockImplementation(async ({ where }) =>
+        [...userEntitlements.values()].filter((entitlement) =>
+          where?.userId ? entitlement.userId === where.userId : true,
+        ),
+      ),
+      createMany: jest.fn().mockImplementation(async ({ data }) => {
+        for (const entry of data) {
+          const id = `${entry.userId}:${entry.entitlementKey}`;
+          userEntitlements.set(id, {
+            id,
+            createdAt: new Date(),
+            updatedAt: new Date(),
+            ...entry,
+          });
+        }
+        return { count: data.length };
+      }),
+      updateMany: jest.fn().mockResolvedValue({ count: 0 }),
+      findUnique: jest.fn().mockResolvedValue(null),
+      upsert: jest.fn().mockImplementation(async ({ create }) => {
+        const id = `${create.userId}:${create.entitlementKey}`;
+        const record = {
+          id,
+          createdAt: new Date(),
+          updatedAt: new Date(),
+          ...create,
+        };
+        userEntitlements.set(id, record);
+        return record;
+      }),
+    },
     track: {
       findMany: jest.fn().mockImplementation(async ({ where }) => {
         return [...tracks.values()]
@@ -448,6 +672,11 @@ function createPrismaMock() {
       artworkAssets,
       uploadSessions,
       libraryEvents,
+      billingCustomers,
+      billingWebhookEvents,
+      userSubscriptions,
+      userOAuthIdentities,
+      userEntitlements,
     },
   };
 }
@@ -458,6 +687,7 @@ describe('Velody API wiring (e2e)', () => {
   let assetsController: AssetsController;
   let accountController: AccountController;
   let artworkController: ArtworkController;
+  let billingController: BillingController;
   let healthController: HealthController;
   let devicesController: DevicesController;
   let libraryController: LibraryController;
@@ -466,6 +696,8 @@ describe('Velody API wiring (e2e)', () => {
   let uploadsService: UploadsService;
   let requestContextService: RequestContextService;
   let deviceAuthService: DeviceAuthService;
+  let authenticationRuntimeService: AuthenticationRuntimeService;
+  let accountAuthGuard: AccountAuthGuard;
   let prismaState: ReturnType<typeof createPrismaMock>['state'];
   let storageRoot: string;
 
@@ -534,6 +766,46 @@ describe('Velody API wiring (e2e)', () => {
     });
   }
 
+  function createAccountExecutionContext(authorizationHeader?: string) {
+    const request = {
+      headers: {
+        authorization: authorizationHeader,
+      },
+    } as any;
+
+    return {
+      request,
+      context: {
+        switchToHttp: () => ({
+          getRequest: () => request,
+        }),
+      } as ExecutionContext,
+    };
+  }
+
+  async function runBillingCheckoutRequest(
+    authorizationHeader: string | undefined,
+    body: unknown,
+  ) {
+    const validationPipe = new ValidationPipe({
+      whitelist: true,
+      forbidNonWhitelisted: true,
+      transform: true,
+    });
+    const { request, context } =
+      createAccountExecutionContext(authorizationHeader);
+    await accountAuthGuard.canActivate(context);
+    const validatedBody = await validationPipe.transform(body, {
+      type: 'body',
+      metatype: CreateBillingCheckoutRequestDto,
+    });
+
+    return billingController.createCheckout(
+      request.accountAuthContext,
+      validatedBody,
+    );
+  }
+
   beforeEach(async () => {
     const prismaSetup = createPrismaMock();
     prismaMock = prismaSetup.prismaMock;
@@ -549,6 +821,13 @@ describe('Velody API wiring (e2e)', () => {
         appVersion: '0.1.0',
         maxUploadSizeBytes: 1024 * 1024 * 1024,
         storageRoot,
+        accountAccessTokenSecret: 'test-account-access-secret',
+        accountAccessTokenTtlSeconds: 900,
+        getPaddleEnvironment: jest.fn().mockReturnValue('sandbox'),
+        getPaddleApiKey: jest.fn().mockReturnValue('paddle-key'),
+        getPaddleWebhookSecret: jest.fn().mockReturnValue('paddle-webhook-secret'),
+        getPaddleProMonthlyPriceId: jest.fn().mockReturnValue('pri_monthly'),
+        getPaddleProYearlyPriceId: jest.fn().mockReturnValue('pri_yearly'),
       })
       .overrideProvider(PrismaService)
       .useValue(prismaMock)
@@ -574,6 +853,7 @@ describe('Velody API wiring (e2e)', () => {
     assetsController = moduleRef.get(AssetsController);
     accountController = moduleRef.get(AccountController);
     artworkController = moduleRef.get(ArtworkController);
+    billingController = moduleRef.get(BillingController);
     healthController = moduleRef.get(HealthController);
     devicesController = moduleRef.get(DevicesController);
     libraryController = moduleRef.get(LibraryController);
@@ -582,6 +862,8 @@ describe('Velody API wiring (e2e)', () => {
     uploadsService = moduleRef.get(UploadsService);
     requestContextService = moduleRef.get(RequestContextService);
     deviceAuthService = moduleRef.get(DeviceAuthService);
+    authenticationRuntimeService = moduleRef.get(AuthenticationRuntimeService);
+    accountAuthGuard = moduleRef.get(AccountAuthGuard);
   });
 
   afterEach(async () => {
@@ -1706,6 +1988,320 @@ describe('Velody API wiring (e2e)', () => {
     ).rejects.toBeInstanceOf(UnauthorizedException);
   });
 
+  it('returns 401 when billing checkout is requested without Authorization', async () => {
+    await expect(
+      runBillingCheckoutRequest(undefined, {
+        plan: BillingCheckoutPlan.PRO_MONTHLY,
+      }),
+    ).rejects.toBeInstanceOf(UnauthorizedException);
+  });
+
+  it('returns 400 when billing checkout plan is invalid', async () => {
+    jest
+      .spyOn(authenticationRuntimeService, 'validateAccessToken')
+      .mockResolvedValueOnce({
+        userId: prismaState.defaultUser.id,
+        sessionId: 'session-1',
+        accessTokenVersion: 1,
+      });
+
+    await expect(
+      runBillingCheckoutRequest('Bearer valid-account-token', {
+        plan: 'INVALID_PLAN',
+      }),
+    ).rejects.toMatchObject({
+      response: {
+        message: expect.arrayContaining([
+          'plan must be one of the following values: PRO_MONTHLY, PRO_YEARLY',
+        ]),
+      },
+    });
+  });
+
+  it('creates a checkout for an authenticated account with a new Paddle customer', async () => {
+    prismaState.userOAuthIdentities.set('identity-1', {
+      id: 'identity-1',
+      userId: prismaState.defaultUser.id,
+      provider: 'GOOGLE',
+      providerSubject: 'subject-1',
+      email: 'owner@example.com',
+      createdAt: new Date(),
+      updatedAt: new Date(),
+    });
+    jest
+      .spyOn(authenticationRuntimeService, 'validateAccessToken')
+      .mockResolvedValueOnce({
+        userId: prismaState.defaultUser.id,
+        sessionId: 'session-1',
+        accessTokenVersion: 1,
+      });
+    const fetchRequest = jest
+      .spyOn(globalThis, 'fetch')
+      .mockResolvedValueOnce({
+        ok: true,
+        json: async () => ({
+          data: {
+            id: 'ctm_new_customer',
+          },
+        }),
+      } as Response)
+      .mockResolvedValueOnce({
+        ok: true,
+        json: async () => ({
+          data: {
+            checkout: {
+              url: 'https://checkout.paddle.test/new-session',
+            },
+          },
+        }),
+      } as Response);
+
+    const response = await runBillingCheckoutRequest(
+      'Bearer valid-account-token',
+      {
+        plan: BillingCheckoutPlan.PRO_MONTHLY,
+      },
+    );
+
+    expect(response).toEqual({
+      plan: BillingCheckoutPlan.PRO_MONTHLY,
+      checkoutUrl: 'https://checkout.paddle.test/new-session',
+    });
+    expect([...prismaState.billingCustomers.values()][0]).toMatchObject({
+      userId: prismaState.defaultUser.id,
+      providerCustomerId: 'ctm_new_customer',
+      provider: 'PADDLE',
+    });
+    expect(prismaState.userSubscriptions.size).toBe(0);
+    expect(prismaState.userEntitlements.size).toBe(0);
+
+    fetchRequest.mockRestore();
+  });
+
+  it('creates a checkout for an authenticated account with an existing Paddle customer', async () => {
+    prismaState.userOAuthIdentities.set('identity-1', {
+      id: 'identity-1',
+      userId: prismaState.defaultUser.id,
+      provider: 'GOOGLE',
+      providerSubject: 'subject-1',
+      email: 'owner@example.com',
+      createdAt: new Date(),
+      updatedAt: new Date(),
+    });
+    prismaState.billingCustomers.set('billing-customer-1', {
+      id: 'billing-customer-1',
+      userId: prismaState.defaultUser.id,
+      provider: 'PADDLE',
+      providerCustomerId: 'ctm_existing_customer',
+      createdAt: new Date(),
+      updatedAt: new Date(),
+    });
+    jest
+      .spyOn(authenticationRuntimeService, 'validateAccessToken')
+      .mockResolvedValueOnce({
+        userId: prismaState.defaultUser.id,
+        sessionId: 'session-1',
+        accessTokenVersion: 1,
+      });
+    const fetchRequest = jest
+      .spyOn(globalThis, 'fetch')
+      .mockResolvedValueOnce({
+        ok: true,
+        json: async () => ({
+          data: {
+            checkout: {
+              url: 'https://checkout.paddle.test/existing-session',
+            },
+          },
+        }),
+      } as Response);
+
+    const response = await runBillingCheckoutRequest(
+      'Bearer valid-account-token',
+      {
+        plan: BillingCheckoutPlan.PRO_YEARLY,
+      },
+    );
+
+    expect(response).toEqual({
+      plan: BillingCheckoutPlan.PRO_YEARLY,
+      checkoutUrl: 'https://checkout.paddle.test/existing-session',
+    });
+    expect(fetchRequest).toHaveBeenCalledTimes(1);
+    expect(prismaState.userSubscriptions.size).toBe(0);
+    expect(prismaState.userEntitlements.size).toBe(0);
+
+    fetchRequest.mockRestore();
+  });
+
+  it('returns 502 when Paddle customer creation fails during billing checkout', async () => {
+    prismaState.userOAuthIdentities.set('identity-1', {
+      id: 'identity-1',
+      userId: prismaState.defaultUser.id,
+      provider: 'GOOGLE',
+      providerSubject: 'subject-1',
+      email: 'owner@example.com',
+      createdAt: new Date(),
+      updatedAt: new Date(),
+    });
+    jest
+      .spyOn(authenticationRuntimeService, 'validateAccessToken')
+      .mockResolvedValueOnce({
+        userId: prismaState.defaultUser.id,
+        sessionId: 'session-1',
+        accessTokenVersion: 1,
+      });
+    const fetchRequest = jest
+      .spyOn(globalThis, 'fetch')
+      .mockResolvedValueOnce({
+        ok: false,
+        json: async () => ({
+          error: {
+            detail: 'Paddle customer error',
+          },
+        }),
+      } as Response);
+
+    await expect(
+      runBillingCheckoutRequest('Bearer valid-account-token', {
+        plan: BillingCheckoutPlan.PRO_MONTHLY,
+      }),
+    ).rejects.toBeInstanceOf(BadGatewayException);
+    expect(prismaState.userSubscriptions.size).toBe(0);
+    expect(prismaState.userEntitlements.size).toBe(0);
+
+    fetchRequest.mockRestore();
+  });
+
+  it('returns 502 when Paddle checkout session creation fails during billing checkout', async () => {
+    prismaState.userOAuthIdentities.set('identity-1', {
+      id: 'identity-1',
+      userId: prismaState.defaultUser.id,
+      provider: 'GOOGLE',
+      providerSubject: 'subject-1',
+      email: 'owner@example.com',
+      createdAt: new Date(),
+      updatedAt: new Date(),
+    });
+    prismaState.billingCustomers.set('billing-customer-1', {
+      id: 'billing-customer-1',
+      userId: prismaState.defaultUser.id,
+      provider: 'PADDLE',
+      providerCustomerId: 'ctm_existing_customer',
+      createdAt: new Date(),
+      updatedAt: new Date(),
+    });
+    jest
+      .spyOn(authenticationRuntimeService, 'validateAccessToken')
+      .mockResolvedValueOnce({
+        userId: prismaState.defaultUser.id,
+        sessionId: 'session-1',
+        accessTokenVersion: 1,
+      });
+    const fetchRequest = jest
+      .spyOn(globalThis, 'fetch')
+      .mockResolvedValueOnce({
+        ok: false,
+        json: async () => ({
+          error: {
+            detail: 'Paddle checkout error',
+          },
+        }),
+      } as Response);
+
+    await expect(
+      runBillingCheckoutRequest('Bearer valid-account-token', {
+        plan: BillingCheckoutPlan.PRO_YEARLY,
+      }),
+    ).rejects.toBeInstanceOf(BadGatewayException);
+    expect(prismaState.userSubscriptions.size).toBe(0);
+    expect(prismaState.userEntitlements.size).toBe(0);
+
+    fetchRequest.mockRestore();
+  });
+
+  it('accepts a valid webhook event without account auth', async () => {
+    const payload = {
+      event_id: 'evt_e2e_valid',
+      event_type: 'transaction.completed',
+      data: {
+        id: 'txn_123',
+      },
+    };
+
+    expect(
+      Reflect.getMetadata(
+        HTTP_CODE_METADATA,
+        BillingController.prototype.ingestWebhook,
+      ),
+    ).toBe(HttpStatus.OK);
+
+    await expect(billingController.ingestWebhook(payload)).resolves.toEqual({
+      ok: true,
+    });
+
+    expect(prismaState.billingWebhookEvents.size).toBe(1);
+    expect([...prismaState.billingWebhookEvents.values()][0]).toMatchObject({
+      provider: 'PADDLE',
+      providerEventId: 'evt_e2e_valid',
+      eventType: 'transaction.completed',
+      payload,
+      status: 'RECEIVED',
+    });
+    expect(prismaState.userSubscriptions.size).toBe(0);
+    expect(prismaState.userEntitlements.size).toBe(0);
+  });
+
+  it('returns ok for duplicate webhook events and does not store them twice', async () => {
+    const payload = {
+      event_id: 'evt_e2e_duplicate',
+      event_type: 'subscription.updated',
+    };
+
+    expect(
+      Reflect.getMetadata(
+        HTTP_CODE_METADATA,
+        BillingController.prototype.ingestWebhook,
+      ),
+    ).toBe(HttpStatus.OK);
+
+    await expect(billingController.ingestWebhook(payload)).resolves.toEqual({
+      ok: true,
+    });
+    await expect(billingController.ingestWebhook(payload)).resolves.toEqual({
+      ok: true,
+    });
+
+    expect(prismaState.billingWebhookEvents.size).toBe(1);
+    expect([...prismaState.billingWebhookEvents.values()][0]).toMatchObject({
+      providerEventId: 'evt_e2e_duplicate',
+      status: 'DUPLICATE',
+    });
+  });
+
+  it('returns 400 for invalid webhook payloads', async () => {
+    await expect(
+      billingController.ingestWebhook({
+        event_type: 'subscription.created',
+      }),
+    ).rejects.toBeInstanceOf(BadRequestException);
+  });
+
+  it('keeps checkout protected while webhook stays public', async () => {
+    await expect(
+      billingController.ingestWebhook({
+        event_id: 'evt_public',
+        event_type: 'subscription.canceled',
+      }),
+    ).resolves.toEqual({ ok: true });
+
+    await expect(
+      runBillingCheckoutRequest(undefined, {
+        plan: BillingCheckoutPlan.PRO_MONTHLY,
+      }),
+    ).rejects.toBeInstanceOf(UnauthorizedException);
+  });
+
   it('rejects invalid or revoked device tokens even when a legacy device id is supplied', async () => {
     const ownerDevice = await devicesController.register({
       platform: 'IPHONE',