支持结账多轮协商与按实际消耗双比例结算

允许双方最多 6 轮修改结账方案；哈夫币按实际消耗与 buyer/seller 比例计价，打超从押金扣，押金不足禁止自动完结并引导人工争议。同步收紧结账明细展示密度。

backend/internal/modules/order/checkout_finalize.go

@@ -26,6 +26,9 @@ func (r *Repository) finalizeCheckout(tx *gorm.DB, order *model.RentalOrder, che
 	}
 
 	settlement := buildCheckoutSettlement(*order, checkout)
+	if settlement.ShortfallCent > 0 {
+		return nil, ErrCheckoutDepositShortfall
+	}
 	if err := appendCheckoutOwnerIncome(tx, order, settlement); err != nil {
 		return nil, err
 	}
@@ -93,6 +96,8 @@ func applyCheckoutSettlement(checkout *model.OrderCheckout, settlement checkoutS
 	checkout.PlatformFeeCent = settlement.PlatformFeeCent
 	checkout.RenterRefundAmountCent = settlement.RenterRefundCent
 	checkout.OwnerIncomeAmountCent = settlement.OwnerIncomeCent
+	checkout.ShortfallCent = settlement.ShortfallCent
+	checkout.OvershootAmountCent = settlement.OvershootAmountCent
 }
 
 func appendCheckoutCompletedNotifications(tx *gorm.DB, order *model.RentalOrder, renterContent string) error {