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/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)