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/test/billing/helpers.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'bun:test';
+
+/**
+ * Helper that replicates the mapWebhookTypeToStatus logic
+ * used in webhook business files for testability.
+ */
+export function mapWebhookTypeToStatus(
+  eventType: 'activated' | 'updated' | 'payment_failed' | 'cancelled'
+): 'ACTIVE' | 'PAST_DUE' | 'CANCELED' | null {
+  switch (eventType) {
+    case 'activated':
+      return 'ACTIVE';
+    case 'updated':
+      return 'ACTIVE';
+    case 'payment_failed':
+      return 'PAST_DUE';
+    case 'cancelled':
+      return 'CANCELED';
+    default:
+      return null;
+  }
+}