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/app.module.ts

@@ -3,6 +3,7 @@ import { AuthModule } from '@modules/auth';
 import { ListingsModule } from '@modules/listings';
 import { SearchModule } from '@modules/search';
 import { NotificationsModule } from '@modules/notifications';
+import { PaymentsModule } from '@modules/payments';
 import { Module } from '@nestjs/common';
 import { APP_GUARD } from '@nestjs/core';
 import { CqrsModule } from '@nestjs/cqrs';
@@ -18,6 +19,7 @@ import { AppController } from './app.controller';
     ListingsModule,
     SearchModule,
     NotificationsModule,
+    PaymentsModule,
 
     // ── Rate Limiting ──
     // Default: 60 requests per 60 seconds per IP