feat: implement Phase 1 payment gateway, real-time SignalR, kitchen-inventory deduction, and order payment flow
- wallet-service: IPaymentGateway abstraction + VN Pay implementation (HMAC-SHA512, sandbox), Payment aggregate root, PaymentsController with create/callback/query endpoints
- order-service: PosHub SignalR hub with Redis backplane + MessagePack, strongly-typed clients, 3 group types (shop/kds/pos), integrated into Create/Pay/Complete/Cancel order handlers
- fnb-engine + inventory-service: Kitchen→Inventory auto-deduction via domain events, HTTP with Polly retry + circuit breaker, idempotency check, graceful degradation on insufficient stock
- order-service: Enhanced PayOrderCommand with 3 flows (cash/card/online), PaymentPending status, WalletServiceClient, CompleteOrderPaymentCommand for gateway callbacks
- POS frontend: Cash/Card/QR payment components wired to real backend, BFF proxy updated
- infra: Traefik routes for fnb-engine, inventory-service, and SignalR WebSocket hub
- ROADMAP.md: Updated with Phase 1 progress tracking
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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