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/handler_error.go

@@ -43,6 +43,10 @@ func writeOrderError(c *gin.Context, err error) {
 		response.Error(c, http.StatusConflict, "checkout_cannot_confirm", "当前结账不能确认")
 	case errors.Is(err, ErrCheckoutCannotCounter):
 		response.Error(c, http.StatusConflict, "checkout_cannot_counter", "当前结账不能修改")
+	case errors.Is(err, ErrCheckoutMaxRounds):
+		response.Error(c, http.StatusConflict, "checkout_max_rounds", "结账协商已达 6 轮上限，请同意当前方案或发起争议由人工处理")
+	case errors.Is(err, ErrCheckoutDepositShortfall):
+		response.Error(c, http.StatusConflict, "checkout_deposit_shortfall", "押金不足以覆盖打超/赔付差额，无法自动完结，请发起争议由人工处理")
 	case errors.Is(err, ErrInvalidCheckoutAmount):
 		response.BadRequest(c, "结账金额不符合规则")
 	case errors.Is(err, ErrPermissionDenied):