Add Paddle webhook ingestion foundation

backend/openapi/velody.openapi.json

@@ -1,6 +1,32 @@
 {
   "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",
@@ -555,6 +581,18 @@
       }
     },
     "schemas": {
+      "BillingWebhookResponseDto": {
+        "type": "object",
+        "properties": {
+          "ok": {
+            "type": "boolean",
+            "example": true
+          }
+        },
+        "required": [
+          "ok"
+        ]
+      },
       "CreateBillingCheckoutRequestDto": {
         "type": "object",
         "properties": {