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/lib/useLiveQuery.ts

@@ -0,0 +1,22 @@
+import { useQuery, type QueryKey } from "@tanstack/react-query";
+
+// Reusable "live" polling query: a thin wrapper over react-query that refetches on an
+// interval and — by leaving refetchIntervalInBackground at its default false — pauses while
+// the tab is unfocused, so it doesn't poll a server nobody's watching. This is the shared
+// live-progress mechanism: the Scheduler dashboard uses it now; the notification bell and the
+// future yt-dlp job queue reuse it rather than each re-implementing polling.
+export function useLiveQuery<T>(
+  key: QueryKey,
+  queryFn: () => Promise<T>,
+  opts: { intervalMs?: number; enabled?: boolean } = {}
+) {
+  const { intervalMs = 4000, enabled = true } = opts;
+  return useQuery({
+    queryKey: key,
+    queryFn,
+    enabled,
+    refetchInterval: enabled ? intervalMs : false,
+    refetchIntervalInBackground: false,
+    staleTime: 0,
+  });
+}