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/status-transitions.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'bun:test';
+import { mapWebhookTypeToStatus } from './helpers';
+
+describe('status-transitions', () => {
+  it('activated maps to ACTIVE', () => {
+    expect(mapWebhookTypeToStatus('activated')).toBe('ACTIVE');
+  });
+
+  it('updated maps to ACTIVE', () => {
+    expect(mapWebhookTypeToStatus('updated')).toBe('ACTIVE');
+  });
+
+  it('payment_failed maps to PAST_DUE', () => {
+    expect(mapWebhookTypeToStatus('payment_failed')).toBe('PAST_DUE');
+  });
+
+  it('cancelled maps to CANCELED', () => {
+    expect(mapWebhookTypeToStatus('cancelled')).toBe('CANCELED');
+  });
+});