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/dto.go

@@ -144,6 +144,12 @@ type CheckoutDTO struct {
 	OrderID                 uint64     `json:"order_id"`
 	InitiatedBy             uint64     `json:"initiated_by"`
 	Status                  string     `json:"status"`
+	RoundCount              int        `json:"round_count"`
+	Turn                    string     `json:"turn"`
+	ProposedBy              uint64     `json:"proposed_by"`
+	MaxRounds               int        `json:"max_rounds"`
+	CanCounter              bool       `json:"can_counter"`
+	CanAccept               bool       `json:"can_accept"`
 	PriceRole               string     `json:"price_role,omitempty"`
 	DisplayAmountCent       int64      `json:"display_amount_cent"`
 	RentAmountCent          *int64     `json:"rent_amount_cent,omitempty"`
@@ -156,6 +162,8 @@ type CheckoutDTO struct {
 	DepositDeductAmountCent int64      `json:"deposit_deduct_amount_cent"`
 	RenterRefundAmountCent  *int64     `json:"renter_refund_amount_cent,omitempty"`
 	OwnerIncomeAmountCent   *int64     `json:"owner_income_amount_cent,omitempty"`
+	ShortfallCent           int64      `json:"shortfall_cent"`
+	OvershootAmountCent     int64      `json:"overshoot_amount_cent"`
 	Content                 string     `json:"content"`
 	EvidenceURLS            []string   `json:"evidence_urls"`
 	OwnerAdjustmentReason   string     `json:"owner_adjustment_reason"`