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/App.tsx

@@ -27,6 +27,7 @@ import Feed from "./components/Feed";
 import Channels, { type ChannelStatusFilter } from "./components/Channels";
 import Playlists from "./components/Playlists";
 import Stats from "./components/Stats";
+import Scheduler from "./components/Scheduler";
 import SettingsPanel from "./components/SettingsPanel";
 import OnboardingWizard from "./components/OnboardingWizard";
 import { shouldAutoOpenOnboarding } from "./lib/onboarding";
@@ -62,7 +63,8 @@ function loadInitialPage(): Page {
     stored === "channels" ||
     stored === "stats" ||
     stored === "playlists" ||
-    stored === "settings"
+    stored === "settings" ||
+    stored === "scheduler"
   )
     return stored;
   return "feed";
@@ -271,6 +273,8 @@ export default function App() {
             />
           ) : page === "stats" && meQuery.data!.role === "admin" ? (
             <Stats />
+          ) : page === "scheduler" && meQuery.data!.role === "admin" ? (
+            <Scheduler />
           ) : page === "playlists" ? (
             <Playlists canWrite={meQuery.data!.can_write} />
           ) : page === "settings" ? (