feat: 后台线下支付订单强制上传支付凭证

在后台创建订单接口中新增 payment_voucher_key 字段，线下支付方式
下该字段为必填，存储对象存储 file_key，便于后续追溯与审计。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/service/order/service.go

@@ -351,6 +351,11 @@ func (s *Service) CreateLegacy(ctx context.Context, req *dto.CreateOrderRequest,
 // 与 CreateH5Order 的核心区别：后台订单不创建待支付状态，wallet 立即扣款，offline 立即激活
 // POST /api/admin/orders
 func (s *Service) CreateAdminOrder(ctx context.Context, req *dto.CreateAdminOrderRequest, buyerType string, buyerID uint) (*dto.OrderResponse, error) {
+	// 线下支付必须上传支付凭证
+	if req.PaymentMethod == model.PaymentMethodOffline && strings.TrimSpace(req.PaymentVoucherKey) == "" {
+		return nil, errors.New(errors.CodeInvalidParam, "线下支付必须上传支付凭证")
+	}
+
 	resolvedCard, resolvedDevice, resolveErr := s.resolveAssetByIdentifier(ctx, req.Identifier)
 	if resolveErr != nil {
 		return nil, resolveErr
@@ -619,6 +624,11 @@ func (s *Service) CreateAdminOrder(ctx context.Context, req *dto.CreateAdminOrde
 		PaymentConfigID:         paymentConfigID,
 	}
 
+	// 线下支付订单写入支付凭证 file_key
+	if req.PaymentMethod == model.PaymentMethodOffline {
+		order.PaymentVoucherKey = strings.TrimSpace(req.PaymentVoucherKey)
+	}
+
 	items := s.buildOrderItems(userID, validationResult.Packages)
 
 	idempotencyKey := buildOrderIdempotencyKey(buyerType, buyerID, orderType, carrierType, carrierID, req.PackageIDs)
@@ -2177,6 +2187,7 @@ func (s *Service) buildOrderResponse(order *model.Order, items []*model.OrderIte
 		PaymentStatus:           order.PaymentStatus,
 		PaymentStatusText:       statusText,
 		PaidAt:                  order.PaidAt,
+		PaymentVoucherKey:       order.PaymentVoucherKey,
 		IsPurchaseOnBehalf:      order.IsPurchaseOnBehalf,
 		CommissionStatus:        order.CommissionStatus,
 		CommissionConfigVersion: order.CommissionConfigVersion,