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_test.go

@@ -192,9 +192,11 @@ func TestSummaryReturnsNormalOrderCountAndScopedExceptionCount(t *testing.T) {
 
 	loc := timeutil.ShanghaiLocation()
 	settledAt := time.Date(2026, 8, 2, 10, 0, 0, 0, loc)
+	// ORD-S3 的结账单终态是 settled（仲裁结案），不纳入"正常结账确认"口径。
 	for _, row := range [][]any{
 		{1, "ORD-S1", 1, 1, 1, 1, settledAt, "owner_wallet", 0, "none", "none", "settled", "completed", settledAt, 300},
 		{2, "ORD-S2", 2, 2, 2, 2, settledAt, "owner_wallet", 0, "none", "none", "settled", "completed", settledAt, 200},
+		{3, "ORD-S3", 3, 3, 3, 3, settledAt, "owner_wallet", 0, "none", "none", "settled", "completed", settledAt, 0},
 	} {
 		if err := db.Exec(`INSERT INTO rental_orders
 			(id, order_no, listing_id, account_id, owner_id, renter_id,
@@ -204,7 +206,11 @@ func TestSummaryReturnsNormalOrderCountAndScopedExceptionCount(t *testing.T) {
 			t.Fatalf("写入订单失败: %v", err)
 		}
 	}
-	for _, row := range [][]any{{1, 1, 1, "accepted", 300, 700}, {2, 2, 1, "accepted", 200, 600}} {
+	for _, row := range [][]any{
+		{1, 1, 1, "accepted", 300, 700},
+		{2, 2, 1, "accepted", 200, 600},
+		{3, 3, 1, "settled", 0, 0},
+	} {
 		if err := db.Exec(`INSERT INTO order_checkouts
 			(id, order_id, initiated_by, status, platform_fee_cent, owner_income_amount_cent)
 			VALUES (?, ?, ?, ?, ?, ?)`, row...).Error; err != nil {
@@ -236,6 +242,7 @@ func TestSummaryReturnsNormalOrderCountAndScopedExceptionCount(t *testing.T) {
 		t.Fatalf("读取每日结算汇总失败: %v", err)
 	}
 	summary := summaryWithPickup(summaryFromDailyRows(nil, settlements, nil), pickup)
+	// ORD-S3 的结账单终态为 settled（仲裁结案），按既有口径不计入"已结算订单数"。
 	if summary.SettledOrderCount != 2 || summary.NormalOrderProfitAmountCent != 500 {
 		t.Fatalf("普通订单数量或利润错误: %+v", summary)
 	}