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/value-objects/index.ts

@@ -0,0 +1 @@
+export { Money } from './money.vo';

apps/api/src/modules/payments/domain/value-objects/money.vo.ts

@@ -0,0 +1,22 @@
+import { ValueObject } from '@modules/shared/domain/value-object';
+import { Result } from '@modules/shared/domain/result';
+
+interface MoneyProps {
+  amountVND: bigint;
+}
+
+export class Money extends ValueObject<MoneyProps> {
+  get value(): bigint {
+    return this.props.amountVND;
+  }
+
+  static create(amountVND: bigint): Result<Money, string> {
+    if (amountVND <= 0n) {
+      return Result.err('Số tiền phải lớn hơn 0');
+    }
+    if (amountVND > 999_999_999_999n) {
+      return Result.err('Số tiền vượt quá giới hạn cho phép');
+    }
+    return Result.ok(new Money({ amountVND }));
+  }
+}