fix(store): GetDisplayState unterscheidet pgx.ErrNoRows; tz-Kommentar im Scheduler

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/backend/internal/scheduler/scheduler.go

@@ -37,6 +37,7 @@ func Run(ctx context.Context, schedules *store.ScreenScheduleStore, screens Scre
 
 // check prüft alle aktiven Zeitpläne und sendet ggf. Befehle.
 func check(ctx context.Context, schedules *store.ScreenScheduleStore, screens ScreenSlugGetter, notifier DisplayCommander) {
+	// Uses process-local timezone — ensure TZ env var is set in the container (e.g. Europe/Berlin).
 	now := time.Now().Format("15:04")
 
 	enabled, err := schedules.ListEnabled(ctx)

server/backend/internal/store/store.go

@@ -353,10 +353,12 @@ func (s *ScreenStore) GetDisplayState(ctx context.Context, screenID string) (str
 	err := s.pool.QueryRow(ctx,
 		`select coalesce(display_state,'unknown')
 		   from screen_status where screen_id = $1`, screenID).Scan(&state)
-	if err != nil {
-		// Kein Eintrag → unbekannt, kein Fehler
+	if errors.Is(err, pgx.ErrNoRows) {
 		return "unknown", nil
 	}
+	if err != nil {
+		return "unknown", err
+	}
 	return state, nil
 }