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/service.go

@@ -19,14 +19,16 @@ var (
 	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")
-	ErrDepositCannotHold       = errors.New("deposit cannot hold")
-	ErrDepositNotHeld          = errors.New("deposit not held")
-	ErrDepositHoldAmountEmpty  = errors.New("deposit hold amount empty")
+	ErrCheckoutCannotSubmit     = errors.New("checkout cannot submit")
+	ErrCheckoutCannotConfirm    = errors.New("checkout cannot confirm")
+	ErrCheckoutCannotCounter    = errors.New("checkout cannot counter")
+	ErrCheckoutMaxRounds        = errors.New("checkout max rounds reached")
+	ErrCheckoutDepositShortfall = errors.New("checkout deposit shortfall")
+	ErrInvalidCheckoutAmount    = errors.New("invalid checkout amount")
+	ErrPermissionDenied         = errors.New("permission denied")
+	ErrDepositCannotHold        = errors.New("deposit cannot hold")
+	ErrDepositNotHeld           = errors.New("deposit not held")
+	ErrDepositHoldAmountEmpty   = errors.New("deposit hold amount empty")
 )
 
 const internalOrderHours = 24