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/en/header.json

@@ -3,6 +3,7 @@
   "searchPlaceholder": "Search your subscriptions…",
   "channelManager": "Channel manager",
   "usageStats": "Usage & stats",
+  "scheduler": "Scheduler",
   "scope": {
     "label": "Feed source",
     "my": "Mine",
@@ -16,6 +17,7 @@
     "channels": "Channels",
     "playlists": "Playlists",
     "stats": "Stats",
+    "scheduler": "Scheduler",
     "settings": "Settings",
     "about": "About",
     "signOut": "Sign out",