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/payment-completed.event.ts

@@ -0,0 +1,14 @@
+import { type DomainEvent } from '@modules/shared/domain/domain-event';
+import { type PaymentProvider } from '@prisma/client';
+
+export class PaymentCompletedEvent implements DomainEvent {
+  readonly eventName = 'payment.completed';
+  readonly occurredAt = new Date();
+
+  constructor(
+    public readonly aggregateId: string,
+    public readonly userId: string,
+    public readonly provider: PaymentProvider,
+    public readonly amountVND: bigint,
+  ) {}
+}