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/provider-resolver.test.ts

@@ -0,0 +1,36 @@
+import { describe, expect, it } from 'bun:test';
+import { mercadopagoBillingProvider } from '@/modules/billing/providers/mercadopago.provider';
+import { stripeBillingProvider } from '@/modules/billing/providers/stripe.provider';
+import { resolveProviderByCountry } from '@/modules/billing/services/provider-resolver.service';
+
+describe('provider-resolver', () => {
+  it('returns MERCADOPAGO for country AR', () => {
+    const provider = resolveProviderByCountry('AR');
+    expect(provider).toBe(mercadopagoBillingProvider);
+  });
+
+  it('returns STRIPE for country BR', () => {
+    const provider = resolveProviderByCountry('BR');
+    expect(provider).toBe(stripeBillingProvider);
+  });
+
+  it('returns STRIPE for country US', () => {
+    const provider = resolveProviderByCountry('US');
+    expect(provider).toBe(stripeBillingProvider);
+  });
+
+  it('returns STRIPE for null country', () => {
+    const provider = resolveProviderByCountry(null);
+    expect(provider).toBe(stripeBillingProvider);
+  });
+
+  it('returns STRIPE for empty country', () => {
+    const provider = resolveProviderByCountry('');
+    expect(provider).toBe(stripeBillingProvider);
+  });
+
+  it('returns STRIPE for any other ISO code', () => {
+    const provider = resolveProviderByCountry('CL');
+    expect(provider).toBe(stripeBillingProvider);
+  });
+});