refactor(fe): migrate plex/messaging/admin/downloads/notifications to qk (R7 S2b)

Extend lib/queryKeys.ts to the remaining ~34 key roots and migrate every
remaining literal (queryKey: [...] AND the positional useLiveQuery keys) in
36 files. All ~200 React Query keys across the app now route through the
factory; zero raw key literals remain outside queryKeys.ts.

The heterogeneous composite plex keys (plex-library/facets/collections/
playlists carry a filters object, a ratingKeys array, or a union/group
discriminator) take a variadic pass-through — the factory centralizes the
ROOT string; single-id keys stay precisely typed. Nullable keys (thread,
playlist, ytSearch) accept null so a null segment is preserved.

Pure substitution — byte-identical key arrays, so TanStack prefix matching
is unchanged. Gate green: typecheck (app+node+e2e), eslint (0 err),
prettier, 68 vitest. Runtime-verified on a fresh dev server: Feed + Plex
modules load, feed/facets/tags/my-status/plex-library/plex-facets/
plex-collections/plex-playlists all 200, full UI renders.

frontend/src/components/NotificationsPanel.tsx

@@ -57,7 +57,7 @@ export default function NotificationsPanel() {
   const { focusChannel: onFocusChannel } = useChannelsActions();
   const qc = useQueryClient();
   const q = useLiveQuery<{ items: AppNotification[]; total: number }>(
-    ["notifications"],
+    qk.notifications(),
     () => api.notifications(),
     { intervalMs: POLL_MS },
   );
@@ -72,8 +72,8 @@ export default function NotificationsPanel() {
   }, []);
 
   const refresh = () => {
-    qc.invalidateQueries({ queryKey: ["notifications"] });
-    qc.invalidateQueries({ queryKey: ["notif-unread"] });
+    qc.invalidateQueries({ queryKey: qk.notifications() });
+    qc.invalidateQueries({ queryKey: qk.notifUnread() });
   };
   const readMut = useMutation({ mutationFn: api.markNotificationRead, onSuccess: refresh });
   const readAllMut = useMutation({ mutationFn: api.markAllNotificationsRead, onSuccess: refresh });