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.

packages/api-contract/src/index.ts

@@ -155,6 +155,20 @@ export {
   adminGeoCountrySchema,
   adminGeoStatsSchema,
 } from './admin';
+export {
+  billingProviderSchema,
+  billingStatusSchema,
+  billingStatusResponseSchema,
+  createCheckoutSchema,
+  checkoutResponseSchema,
+} from './billing';
+export type {
+  BillingProvider,
+  BillingStatus,
+  BillingStatusResponse,
+  CreateCheckoutInput,
+  CheckoutResponse,
+} from './billing';
 export type {
   AdminComplexListItem,
   AdminComplexStats,