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/prisma/complex.prisma

@@ -15,6 +15,7 @@ model Complex {
   invitations     ComplexInvitation[]
   courts          Court[]
   recurringGroups RecurringBookingGroup[]
+  billing         ComplexBilling?
 
   @@index([planCode])
   @@index([complexSlug])