fix: Zeitformat-Validierung + pgx.ErrNoRows in ScreenScheduleStore.Get

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

server/backend/internal/httpapi/manage/schedule.go

@@ -3,6 +3,7 @@ package manage
 import (
 	"encoding/json"
 	"net/http"
+	"time"
 
 	"git.az-it.net/az/morz-infoboard/server/backend/internal/store"
 )
@@ -30,6 +31,18 @@ func HandleUpdateSchedule(screens *store.ScreenStore, schedules *store.ScreenSch
 			http.Error(w, "invalid JSON", http.StatusBadRequest)
 			return
 		}
+		if body.PowerOnTime != "" {
+			if _, err := time.Parse("15:04", body.PowerOnTime); err != nil {
+				http.Error(w, "invalid power_on_time format (use HH:MM)", http.StatusBadRequest)
+				return
+			}
+		}
+		if body.PowerOffTime != "" {
+			if _, err := time.Parse("15:04", body.PowerOffTime); err != nil {
+				http.Error(w, "invalid power_off_time format (use HH:MM)", http.StatusBadRequest)
+				return
+			}
+		}
 
 		if err := schedules.Upsert(r.Context(), &store.ScreenSchedule{
 			ScreenID:        screen.ID,

server/backend/internal/store/store.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"time"
 
+	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
@@ -653,9 +654,12 @@ func (s *ScreenScheduleStore) Get(ctx context.Context, screenID string) (*Screen
 		`select screen_id, schedule_enabled, power_on_time, power_off_time
 		   from screen_schedules where screen_id = $1`, screenID).
 		Scan(&sc.ScreenID, &sc.ScheduleEnabled, &sc.PowerOnTime, &sc.PowerOffTime)
-	if err != nil {
+	if errors.Is(err, pgx.ErrNoRows) {
 		return &ScreenSchedule{ScreenID: screenID}, nil
 	}
+	if err != nil {
+		return nil, err
+	}
 	return &sc, nil
 }