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/billing.prisma

@@ -0,0 +1,52 @@
+enum BillingProvider {
+  STRIPE
+  MERCADOPAGO
+  PAYPAL
+}
+
+enum BillingStatus {
+  TRIAL
+  ACTIVE
+  PAST_DUE
+  CANCELED
+  SUSPENDED
+}
+
+model ComplexBilling {
+  complexId              String         @id @map("complex_id") @db.Uuid
+  status                 BillingStatus  @default(TRIAL)
+  planCode               String         @map("plan_code") @db.VarChar(10)
+  currency               String         @db.VarChar(3)
+  provider               BillingProvider?
+  providerCustomerId     String?        @map("provider_customer_id")
+  providerSubscriptionId String?        @map("provider_subscription_id")
+  providerPreapprovalId  String?        @map("provider_preapproval_id")
+  currentPeriodStart     DateTime?      @map("current_period_start")
+  currentPeriodEnd       DateTime?      @map("current_period_end")
+  trialEndsAt            DateTime?      @map("trial_ends_at")
+  canceledAt             DateTime?      @map("canceled_at")
+  suspendedAt            DateTime?      @map("suspended_at")
+  createdAt              DateTime       @default(now()) @map("created_at")
+  updatedAt              DateTime       @updatedAt @map("updated_at")
+  complex                Complex        @relation(fields: [complexId], references: [id], onDelete: Cascade)
+  events                 BillingEvent[]
+
+  @@map("complex_billings")
+}
+
+model BillingEvent {
+  id               String         @id @db.Uuid
+  complexId        String         @map("complex_id") @db.Uuid
+  eventType        String         @map("event_type") @db.VarChar(100)
+  provider         BillingProvider
+  providerEventId  String?        @unique @map("provider_event_id")
+  providerData     Json?          @map("provider_data")
+  previousStatus   BillingStatus? @map("previous_status")
+  newStatus        BillingStatus? @map("new_status")
+  processedAt      DateTime       @default(now()) @map("processed_at")
+  billing          ComplexBilling @relation(fields: [complexId], references: [complexId])
+
+  @@index([provider, providerEventId])
+  @@index([complexId])
+  @@map("billing_events")
+}