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/PlexCollectionEditor.tsx

@@ -1,4 +1,5 @@
 import { useMemo, useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Check, Plus } from "lucide-react";
@@ -35,7 +36,7 @@ export default function PlexCollectionEditor({
   const [busy, setBusy] = useState<string | null>(null); // collection id currently mutating
 
   const listQ = useQuery({
-    queryKey: ["plex-collections", library],
+    queryKey: qk.plexCollections(library),
     queryFn: () => api.plexCollections(library),
   });
   const editable = useMemo(
@@ -54,9 +55,9 @@ export default function PlexCollectionEditor({
   const shown = term ? editable.filter((c) => c.title.toLowerCase().includes(term)) : editable;
 
   const refresh = () => {
-    qc.invalidateQueries({ queryKey: ["plex-collections"] });
-    qc.invalidateQueries({ queryKey: ["plex-item", item.id] });
-    qc.invalidateQueries({ queryKey: ["plex-library"] });
+    qc.invalidateQueries({ queryKey: qk.plexCollections() });
+    qc.invalidateQueries({ queryKey: qk.plexItem(item.id) });
+    qc.invalidateQueries({ queryKey: qk.plexLibrary() });
     onChanged();
   };