refactor(order): 结账单状态常量化并锁定仲裁结单统计口径

仲裁结案会把结账单置为终态 settled，但该状态一直是裸字符串；同时 adminfinance 按日结算聚合只按
accepted 过滤，容易被误读成"仲裁单掉出统计"。本次只做卫生与口径固化，不改任何行为：
- order 新增 CheckoutStatus{Submitted,Countered,Accepted,Disputed,Settled}，包内沿用非导出别名
- dispute 的仲裁结案与争议取消分支改用常量；distribution 不能反向依赖 order，本地声明同名常量
- adminfinance 聚合里的 oc.status = 'accepted' 改为常量绑定，并注释说明仲裁单不计入是有意口径
- 新增 distribution 结账单退款重算用例（该分支原先无覆盖）：accepted/settled 按结账单实际租金，
  submitted 不被采用；dashboard 用例补一张 settled 结账单，钉住"不计入已结算订单数"

backend/internal/modules/dispute/arbitration.go

@@ -375,7 +375,7 @@ func applyArbitrationActualSettlement(order *model.RentalOrder, checkout *model.
 	checkout.OwnerIncomeAmountCent = settlement.OwnerIncomeCent
 	checkout.ShortfallCent = settlement.ShortfallCent
 	checkout.OvershootAmountCent = settlement.OvershootAmountCent
-	checkout.Status = "settled"
+	checkout.Status = ordermodule.CheckoutStatusSettled
 }
 
 func isPlatformManagedOrder(order model.RentalOrder) bool {

backend/internal/modules/dispute/cancel.go

@@ -6,6 +6,7 @@ import (
 
 	"hfb_sys/backend/internal/model"
 	"hfb_sys/backend/internal/modules/notification"
+	ordermodule "hfb_sys/backend/internal/modules/order"
 
 	"gorm.io/gorm"
 	"gorm.io/gorm/clause"
@@ -145,9 +146,9 @@ func restoreCheckoutDispute(tx *gorm.DB, row *model.Dispute, order *model.Rental
 	}
 	checkout.Status = row.PreviousCheckoutStatus
 	if checkout.Status == "" {
-		checkout.Status = "submitted"
+		checkout.Status = ordermodule.CheckoutStatusSubmitted
 		if order.Status == "pending_checkout_accept" {
-			checkout.Status = "countered"
+			checkout.Status = ordermodule.CheckoutStatusCountered
 		}
 	}
 	checkout.RenterRejectedAt = nil