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/Stats.tsx

@@ -57,7 +57,7 @@ function Overview({ me }: { me: Me }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
   const status = useQuery({ queryKey: qk.myStatus(), queryFn: api.myStatus });
-  const usage = useQuery({ queryKey: ["my-usage"], queryFn: api.myUsage });
+  const usage = useQuery({ queryKey: qk.myUsage(), queryFn: api.myUsage });
   const s = status.data;
   const syncSubs = useMutation({
     mutationFn: () => api.syncSubscriptions(),
@@ -200,7 +200,7 @@ function SystemStats() {
   const { t } = useTranslation();
   const qc = useQueryClient();
   const [days, setDays] = useState<number>(30);
-  const q = useQuery({ queryKey: ["admin-quota", days], queryFn: () => api.adminQuota(days) });
+  const q = useQuery({ queryKey: qk.adminQuota(days), queryFn: () => api.adminQuota(days) });
   const status = useQuery({ queryKey: qk.myStatus(), queryFn: api.myStatus });
   const pauseResume = useMutation({
     mutationFn: (paused: boolean) => (paused ? api.resumeSync() : api.pauseSync()),