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/order/constants.go

@@ -54,6 +54,16 @@ const (
 	checkoutStatusAccepted  = "accepted"
 	checkoutStatusDisputed  = "disputed"
 
+	// 结账单状态的跨模块导出别名：dispute 仲裁结案写 settled，adminfinance 统计按
+	// accepted 过滤，这些跨包位置不应再出现裸字符串。包内沿用上面的非导出名。
+	// CheckoutStatusSettled 是仲裁终态（写回实际结算金额后置位），不属于
+	// hasOpenCheckout 的"待结"集合——仲裁只发生在 completed 订单上。
+	CheckoutStatusSubmitted = checkoutStatusSubmitted
+	CheckoutStatusCountered = checkoutStatusCountered
+	CheckoutStatusAccepted  = checkoutStatusAccepted
+	CheckoutStatusDisputed  = checkoutStatusDisputed
+	CheckoutStatusSettled   = "settled"
+
 	checkoutTurnOwner  = "owner"
 	checkoutTurnRenter = "renter"