feat(scheduler): live admin Scheduler dashboard + reusable poll hook (HU/EN/DE)

New admin Scheduler page (left-nav entry) with a live, self-refreshing view of
job activity, queued work and quota. Polling is factored into a reusable
useLiveQuery hook (pauses when the tab is unfocused) that the notification bell
and future yt-dlp job queue will reuse instead of re-implementing.

frontend/src/i18n/locales/de/header.json

@@ -3,6 +3,7 @@
   "searchPlaceholder": "Deine Abos durchsuchen…",
   "channelManager": "Kanalverwaltung",
   "usageStats": "Nutzung & Statistik",
+  "scheduler": "Planer",
   "scope": {
     "label": "Feed-Quelle",
     "my": "Meine",
@@ -16,6 +17,7 @@
     "channels": "Kanäle",
     "playlists": "Wiedergabelisten",
     "stats": "Statistik",
+    "scheduler": "Planer",
     "settings": "Einstellungen",
     "about": "Über",
     "signOut": "Abmelden",