fix(认证): 订阅兑换失效认证缓存

订阅兑换后同步失效认证缓存避免授权快照滞后
补充单测覆盖订阅兑换的失效场景

测试: go test ./... -tags=unit

backend/internal/service/redeem_service.go

@@ -348,6 +348,9 @@ func (s *RedeemService) invalidateRedeemCaches(ctx context.Context, userID int64
 			return
 		}
 	case RedeemTypeSubscription:
+		if s.authCacheInvalidator != nil {
+			s.authCacheInvalidator.InvalidateAuthCacheByUserID(ctx, userID)
+		}
 		if s.billingCacheService == nil {
 			return
 		}