fix: 修复平台自营资源（未分配代理）无法线下下单的问题

offline 支付分支新增平台自营子场景判断：
- 资源 shopID 为空时（未分配给任何代理商），使用零售价直接创建订单
- 资源 shopID 不为空时（属于代理商），走原有平台代购逻辑

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/service/order/service.go

@@ -398,7 +398,25 @@ func (s *Service) CreateAdminOrder(ctx context.Context, req *dto.CreateAdminOrde
 
 	// 根据支付方式分别处理
 	if req.PaymentMethod == model.PaymentMethodOffline {
-		// ==== 场景 1：平台代购（offline）====
+		// ==== 场景 1：offline（线下支付）====
+		isPlatformOwned := resourceShopID == nil || *resourceShopID == 0
+
+		if isPlatformOwned {
+			// ==== 子场景 1.1：平台自营（资源未分配给代理商）====
+			// 平台直接销售给终端用户，无代理商参与，使用零售价，无需查询分配配置
+			orderBuyerType = model.BuyerTypePersonal
+			orderBuyerID = 0
+			paymentMethod = model.PaymentMethodOffline
+			paymentStatus = model.PaymentStatusPaid
+			paidAt = &now
+			isPurchaseOnBehalf = false
+			sellerCostPrice = 0
+			operatorID = nil
+			operatorType = constants.OwnerTypePlatform
+			purchaseRole = model.PurchaseRoleSelfPurchase
+			actualPaidAmount = nil
+		} else {
+			// ==== 子场景 1.2：平台代购（资源属于代理商，平台代为购买）====
 			purchaseBuyerID, buyerCostPrice, purchasePaidAt, err := s.resolvePurchaseOnBehalfInfo(ctx, validationResult)
 			if err != nil {
 				return nil, err
@@ -417,6 +435,7 @@ func (s *Service) CreateAdminOrder(ctx context.Context, req *dto.CreateAdminOrde
 			operatorType = constants.OwnerTypePlatform
 			purchaseRole = model.PurchaseRolePurchasedByPlatform
 			actualPaidAmount = nil
+		}
 
 	} else if req.PaymentMethod == model.PaymentMethodWallet {
 		// ==== 场景 2：代理钱包支付（wallet）====