test(admin): add t.Parallel() to exceptional hours test

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/admin/today_test.go

@@ -820,6 +820,7 @@ func TestAdminToday_WeekSummary_TomorrowClosed(t *testing.T) {
 // a closed day, even when default working_hours says today is open.
 // This tests the column name fix: monday_week_start → week_start.
 func TestAdminToday_ExceptionalHours_ClosedDay(t *testing.T) {
+	t.Parallel()
 	ctx, tx := testutils.SetupTestTx(t)
 	now := time.Now()
 	todayStart := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, now.Location())