feat: 绑定新邮箱时创建独立邮箱用户并转移订阅，而非挂在设备用户上

- bindEmailWithVerificationLogic: 新邮箱路径改为创建独立 email user + joinFamily
- familyBindingHelper: clearMemberSubscribes → transferMemberSubscribesToOwner，订阅转移给 owner 而非删除
- accountMergeHelper: 同步更新调用点

Co-Authored-By: claude-flow <ruv@ruv.net>

internal/logic/public/user/familyBindingHelper.go

@@ -182,7 +182,7 @@ func (h *familyBindingHelper) joinFamily(ownerUserId, memberUserId int64, source
 			}
 		}
 
-		removedSubscribes, err = clearMemberSubscribes(tx, memberUserId)
+		removedSubscribes, err = transferMemberSubscribesToOwner(tx, memberUserId, ownerUserId)
 		if err != nil {
 			return err
 		}
@@ -199,7 +199,7 @@ func (h *familyBindingHelper) joinFamily(ownerUserId, memberUserId int64, source
 	return result, nil
 }
 
-func clearMemberSubscribes(tx *gorm.DB, memberUserId int64) ([]user.Subscribe, error) {
+func transferMemberSubscribesToOwner(tx *gorm.DB, memberUserId, ownerUserId int64) ([]user.Subscribe, error) {
 	var subscribes []user.Subscribe
 	if err := tx.Model(&user.Subscribe{}).
 		Where("user_id = ?", memberUserId).
@@ -209,8 +209,10 @@ func clearMemberSubscribes(tx *gorm.DB, memberUserId int64) ([]user.Subscribe, e
 	if len(subscribes) == 0 {
 		return nil, nil
 	}
-	if err := tx.Where("user_id = ?", memberUserId).Delete(&user.Subscribe{}).Error; err != nil {
-		return nil, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseUpdateError), "delete member subscribe list failed")
+	if err := tx.Model(&user.Subscribe{}).
+		Where("user_id = ?", memberUserId).
+		Update("user_id", ownerUserId).Error; err != nil {
+		return nil, errors.Wrapf(xerr.NewErrCode(xerr.DatabaseUpdateError), "transfer member subscribes to owner failed")
 	}
 	return subscribes, nil
 }