feat(billing): implement billing module with Stripe and Mercado Pago integration

- Add Stripe and Mercado Pago providers for handling subscriptions.
- Create billing repository and access services for managing billing records.
- Implement webhook handlers for processing events from Stripe and Mercado Pago.
- Add routes for checkout, canceling subscriptions, and retrieving billing status.
- Introduce billing status management with appropriate state transitions.
- Create tests for billing functionalities including webhook idempotency and provider resolution.
- Document the billing module architecture, environment variables, and API endpoints.

apps/backend/package.json

@@ -24,12 +24,14 @@
     "dotenv": "^17.4.1",
     "hono": "4.12.10",
     "mailtrap": "^4.5.1",
+    "mercadopago": "^3.1.0",
     "nodemailer": "^8.0.5",
     "pg": "^8.20.0",
     "pino": "10.3.1",
     "pino-pretty": "13.1.3",
     "pino-std-serializers": "7.1.0",
     "prisma": "^7",
+    "stripe": "^22.2.2",
     "uuid": "^13.0.0"
   },
   "devDependencies": {