feat(merchant): 新建商户自动复制自营商户的 27 个默认商品

- CreateMerchant 事务内调用 copyDefaultProducts，将自营商户全部可售商品复制给新商户
- 新增 model.MerchantCodeSelfOperated 常量，消除散落的 self-operated 魔法字符串
- tenant.go 替换魔法字符串为常量
- 实测：新建商户后自动获得 27 个商品，价格/货币/库存/关联商品均正确

backend/internal/model/merchant.go

@@ -10,6 +10,9 @@ const (
 	MerchantStatusActive   = "active"
 	MerchantStatusDisabled = "disabled"
 
+	// MerchantCodeSelfOperated 是平台自营商户编码，作为默认商品目录的模板来源。
+	MerchantCodeSelfOperated = "self-operated"
+
 	MerchantFeatureProducts  = "products"
 	MerchantFeatureOrders    = "orders"
 	MerchantFeatureWallet    = "wallet"

backend/internal/service/merchant.go

@@ -103,6 +103,9 @@ func (s *MerchantService) CreateMerchant(in CreateMerchantInput, actorUserID uin
 		}).Error; err != nil {
 			return err
 		}
+		if err := copyDefaultProducts(tx, merchant.ID); err != nil {
+			return err
+		}
 		return writeAudit(tx, &merchant.ID, &actorUserID, nil, "merchant.create", "merchant", fmt.Sprint(merchant.ID), map[string]string{"code": merchant.Code})
 	})
 	if err != nil {
@@ -532,6 +535,45 @@ func (s *MerchantService) UpdateAPIClientStatus(merchantID, id uint, status stri
 	})
 }
 
+// copyDefaultProducts 将自营商户的全部可售商品复制给新建商户，作为默认商品目录。
+// 自营商户（self-operated）充当平台默认商品模板，新商户开箱即用。
+// 使用 OnConflict DoNothing 保证幂等：即使重复调用也不会报唯一索引冲突。
+func copyDefaultProducts(tx *gorm.DB, merchantID uint) error {
+	var selfMerchant model.Merchant
+	if err := tx.Where("code = ?", model.MerchantCodeSelfOperated).First(&selfMerchant).Error; err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil // 自营商户不存在时跳过，不阻断建商户
+		}
+		return err
+	}
+	if selfMerchant.ID == merchantID {
+		return nil // 自营商户自身无需复制
+	}
+	var templates []model.MerchantProduct
+	if err := tx.Where("merchant_id = ?", selfMerchant.ID).Find(&templates).Error; err != nil {
+		return err
+	}
+	if len(templates) == 0 {
+		return nil
+	}
+	products := make([]model.MerchantProduct, 0, len(templates))
+	for _, t := range templates {
+		products = append(products, model.MerchantProduct{
+			MerchantID:        merchantID,
+			ProductID:         t.ProductID,
+			SKU:               t.SKU,
+			DisplayName:       t.DisplayName,
+			PriceAmount:       t.PriceAmount,
+			CostAmount:        t.CostAmount,
+			Currency:          t.Currency,
+			Stock:             t.Stock,
+			Status:            t.Status,
+			FulfillmentConfig: t.FulfillmentConfig,
+		})
+	}
+	return tx.Clauses(clause.OnConflict{DoNothing: true}).Create(&products).Error
+}
+
 func ensureProduct(tx *gorm.DB, in CreateMerchantProductInput) (*model.Product, error) {
 	if in.ProductCode != "" {
 		var product model.Product

backend/internal/service/tenant.go

@@ -47,7 +47,7 @@ func (s *TenantService) ResolveMember(userID uint, merchantRef string) (*model.M
 
 func (s *TenantService) EnsureSelfMember(userID uint, role string, status int) error {
 	var merchant model.Merchant
-	if err := s.db.Where("code = ?", "self-operated").First(&merchant).Error; err != nil {
+	if err := s.db.Where("code = ?", model.MerchantCodeSelfOperated).First(&merchant).Error; err != nil {
 		return err
 	}
 	return s.EnsureMember(merchant.ID, userID, role, status, role == model.MemberRoleOwner)