实现结账流程：替换旧归还逻辑为完整结账-修正-争议流程

- 新增 order_checkouts 表，支持结账明细（消耗/押金扣除/退回/号主入账）
- 租客发起结账 → 号主确认 → 已完成（正常路径）
- 号主修改结账 → 租客确认修正 → 已完成（修正路径）
- 结账阶段任一方发起争议 → 后台仲裁 → 已完成/已关闭/异常（争议路径）
- 争议模块适配结账争议类型，仲裁结果新增 mark_abnormal
- 超时任务适配新的 pending_checkout_confirm 状态
- 前端结账明细面板、发起结账/确认/修正/拒绝表单全部实现
- 移除旧的 SubmitReturn/ConfirmReturn 接口

backend/internal/modules/order/service.go

@@ -10,8 +10,11 @@ var (
 	ErrOrderCannotCancel     = errors.New("order cannot cancel")
 	ErrOrderCannotHandoff    = errors.New("order cannot handoff")
 	ErrOrderCannotReceive    = errors.New("order cannot receive")
-	ErrOrderCannotReturn     = errors.New("order cannot return")
 	ErrOrderCannotComplete   = errors.New("order cannot complete")
+	ErrCheckoutCannotSubmit  = errors.New("checkout cannot submit")
+	ErrCheckoutCannotConfirm = errors.New("checkout cannot confirm")
+	ErrCheckoutCannotCounter = errors.New("checkout cannot counter")
+	ErrInvalidCheckoutAmount = errors.New("invalid checkout amount")
 	ErrPermissionDenied      = errors.New("permission denied")
 )
 
@@ -66,21 +69,38 @@ func (s *Service) HandoffRecords(userID uint64, orderID uint64) ([]HandoffRecord
 	return s.repo.HandoffRecords(userID, orderID)
 }
 
-func (s *Service) SubmitReturn(userID uint64, orderID uint64, req SubmitReturnRequest) (*HandoffRecordDTO, error) {
+func (s *Service) SubmitCheckout(userID uint64, orderID uint64, req SubmitCheckoutRequest) (*HandoffRecordDTO, error) {
 	if s.repo == nil {
 		return nil, ErrDependencyUnavailable
 	}
 	if orderID == 0 || req.Content == "" {
-		return nil, ErrOrderCannotReturn
+		return nil, ErrCheckoutCannotSubmit
 	}
-	return s.repo.SubmitReturn(userID, orderID, req)
+	return s.repo.SubmitCheckout(userID, orderID, req)
 }
 
-func (s *Service) ConfirmReturn(userID uint64, orderID uint64) error {
+func (s *Service) ConfirmCheckout(userID uint64, orderID uint64) error {
 	if s.repo == nil {
 		return ErrDependencyUnavailable
 	}
-	return s.repo.ConfirmReturn(userID, orderID)
+	return s.repo.ConfirmCheckout(userID, orderID)
+}
+
+func (s *Service) CounterCheckout(userID uint64, orderID uint64, req CounterCheckoutRequest) (*CheckoutDTO, error) {
+	if s.repo == nil {
+		return nil, ErrDependencyUnavailable
+	}
+	if orderID == 0 || req.Reason == "" {
+		return nil, ErrCheckoutCannotCounter
+	}
+	return s.repo.CounterCheckout(userID, orderID, req)
+}
+
+func (s *Service) AcceptCheckout(userID uint64, orderID uint64) error {
+	if s.repo == nil {
+		return ErrDependencyUnavailable
+	}
+	return s.repo.AcceptCheckout(userID, orderID)
 }
 
 func (s *Service) ListForUser(userID uint64) ([]OrderDTO, error) {