feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/internal/service/payment_order_expiry_service.go

@@ -0,0 +1,73 @@
+package service
+
+import (
+	"context"
+	"log/slog"
+	"sync"
+	"time"
+)
+
+const expiryCheckTimeout = 30 * time.Second
+
+// PaymentOrderExpiryService periodically expires timed-out payment orders.
+type PaymentOrderExpiryService struct {
+	paymentSvc *PaymentService
+	interval   time.Duration
+	stopCh     chan struct{}
+	stopOnce   sync.Once
+	wg         sync.WaitGroup
+}
+
+func NewPaymentOrderExpiryService(paymentSvc *PaymentService, interval time.Duration) *PaymentOrderExpiryService {
+	return &PaymentOrderExpiryService{
+		paymentSvc: paymentSvc,
+		interval:   interval,
+		stopCh:     make(chan struct{}),
+	}
+}
+
+func (s *PaymentOrderExpiryService) Start() {
+	if s == nil || s.paymentSvc == nil || s.interval <= 0 {
+		return
+	}
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+		ticker := time.NewTicker(s.interval)
+		defer ticker.Stop()
+
+		s.runOnce()
+		for {
+			select {
+			case <-ticker.C:
+				s.runOnce()
+			case <-s.stopCh:
+				return
+			}
+		}
+	}()
+}
+
+func (s *PaymentOrderExpiryService) Stop() {
+	if s == nil {
+		return
+	}
+	s.stopOnce.Do(func() {
+		close(s.stopCh)
+	})
+	s.wg.Wait()
+}
+
+func (s *PaymentOrderExpiryService) runOnce() {
+	ctx, cancel := context.WithTimeout(context.Background(), expiryCheckTimeout)
+	defer cancel()
+
+	expired, err := s.paymentSvc.ExpireTimedOutOrders(ctx)
+	if err != nil {
+		slog.Error("[PaymentOrderExpiry] failed to expire orders", "error", err)
+		return
+	}
+	if expired > 0 {
+		slog.Info("[PaymentOrderExpiry] expired timed-out orders", "count", expired)
+	}
+}