Add Paddle checkout foundation

backend/openapi/velody.openapi.json

@@ -1,6 +1,51 @@
 {
   "openapi": "3.0.0",
   "paths": {
+    "/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 +551,45 @@
         "scheme": "bearer",
         "bearerFormat": "Bearer",
         "type": "http",
-        "description": "Device access token"
+        "description": "Device access token or account access token"
       }
     },
     "schemas": {
+      "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": {