feat(payments): implement Payments module with VNPay, MoMo, ZaloPay integration

Implement complete payment processing module following DDD + CQRS patterns:

- Domain layer: PaymentEntity aggregate, Money value object, domain events
- Infrastructure: PrismaPaymentRepository, VnpayService, MomoService, ZalopayService
- PaymentGatewayFactory pattern for provider abstraction
- CQRS Commands: CreatePayment, HandleCallback, RefundPayment
- CQRS Queries: GetPaymentStatus, ListTransactions
- Callback/webhook endpoints with signature verification and idempotency
- 23 unit tests covering domain, VNPay service, and gateway factory

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/payments/domain/events/index.ts

@@ -0,0 +1,3 @@
+export { PaymentCreatedEvent } from './payment-created.event';
+export { PaymentCompletedEvent } from './payment-completed.event';
+export { PaymentFailedEvent } from './payment-failed.event';