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",

frontend/src/i18n/locales/en/scheduler.json

@@ -0,0 +1,46 @@
+{
+  "title": "Background scheduler",
+  "loading": "Loading scheduler…",
+  "running": "Running",
+  "paused": "Paused",
+  "notHere": "Not running in this instance",
+  "notHereNote": "The scheduler runs on the server. This instance shows the shared queue and quota, but live job activity appears only where the scheduler runs.",
+  "updating": "updating…",
+  "pause": "Pause",
+  "resume": "Resume",
+  "pauseHint": "Pause or resume all background sync for the whole app (every user).",
+  "toggleFailed": "Couldn't change the scheduler state",
+  "jobsTitle": "Jobs",
+  "queueTitle": "Work queued",
+  "quotaTitle": "Shared API quota",
+  "everyMin": "every {{count}} min",
+  "runningNow": "Running now…",
+  "lastRun": "Last run {{time}}",
+  "neverRun": "Hasn't run yet this session",
+  "active": "active",
+  "nextRun": "next",
+  "quotaUsed": "{{used}} / {{budget}} units used today",
+  "quotaRemaining": "{{formatted}} left",
+  "quotaNote": "Shared YouTube Data API budget; resets at midnight US Pacific. Backfill yields to leave headroom for interactive use.",
+  "jobs": {
+    "rss_poll": "RSS poll (new uploads)",
+    "enrich": "Enrichment + live refresh",
+    "backfill": "Backfill (recent + full history)",
+    "autotag": "Auto-tagging",
+    "shorts": "Shorts classification",
+    "subscriptions": "Subscription resync",
+    "playlist_sync": "YouTube playlist sync"
+  },
+  "queue": {
+    "recentPending": "Channels to sync",
+    "recentPendingHint": "Channels whose recent uploads haven't been fetched yet.",
+    "deepPending": "Full-history pending",
+    "deepPendingHint": "Channels opted into full-history backfill that aren't finished yet.",
+    "enrichPending": "Videos to enrich",
+    "enrichPendingHint": "Videos awaiting details (duration, stats, live/short classification).",
+    "shortsPending": "Shorts to classify",
+    "shortsPendingHint": "Enriched videos still awaiting the Shorts probe.",
+    "liveRefresh": "Live to refresh",
+    "liveRefreshHint": "Live/upcoming videos (and just-ended streams without a duration yet) re-checked until they settle."
+  }
+}