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/adminfinance/dashboard.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"sync"
 
+	ordermodule "hfb_sys/backend/internal/modules/order"
 	"hfb_sys/backend/internal/timeutil"
 )
 
@@ -442,7 +443,9 @@ func (r *Repository) dailySettlements(ctx context.Context, query DashboardQuery)
 						ELSE 0
 					END), 0) AS financial_exception_count,
 					COUNT(ro.id) AS settled_order_count`, settlementDiffThresholdCent, refundBizTypes(), []string{"refunding", "failed"}).
-		Joins("JOIN order_checkouts AS oc ON oc.order_id = ro.id AND oc.status = 'accepted'").
+		// 统计口径：只算"正常结账确认"的订单。仲裁结案的结账单终态是 settled
+		// （由 dispute 模块写入），不纳入本指标，此处保持既有口径不变。
+		Joins("JOIN order_checkouts AS oc ON oc.order_id = ro.id AND oc.status = ?", ordermodule.CheckoutStatusAccepted).
 		Joins("LEFT JOIN (?) AS w ON w.order_id = ro.id", ownerWalletIncomeForSettledOrdersSubquery(db, query)).
 		Where("ro.settled_at >= ? AND ro.settled_at <= ?", query.StartDate, query.EndDate).
 		Group("DATE_FORMAT(ro.settled_at, '%Y-%m-%d')").