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/PlexPlaylistView.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useMemo, useState, type ReactNode } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import {
@@ -375,7 +376,7 @@ export default function PlexPlaylistView({
   const [collapsedSeasons, setCollapsedSeasons] = useState<Set<string>>(new Set());
 
   const q = useQuery({
-    queryKey: ["plex-playlist", playlistId],
+    queryKey: qk.plexPlaylist(playlistId),
     queryFn: () => api.plexPlaylist(playlistId),
   });
   // Local optimistic copy so drag/remove are instant; re-synced whenever the query returns.
@@ -387,8 +388,8 @@ export default function PlexPlaylistView({
   const order = useMemo(() => items.map((i) => i.id), [items]);
 
   const invalidate = () => {
-    qc.invalidateQueries({ queryKey: ["plex-playlist", playlistId] });
-    qc.invalidateQueries({ queryKey: ["plex-playlists"] });
+    qc.invalidateQueries({ queryKey: qk.plexPlaylist(playlistId) });
+    qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
   };
 
   function commit(next: PlexCard[]) {
@@ -398,7 +399,7 @@ export default function PlexPlaylistView({
         playlistId,
         next.map((i) => i.id),
       )
-      .then(() => qc.invalidateQueries({ queryKey: ["plex-playlists"] }));
+      .then(() => qc.invalidateQueries({ queryKey: qk.plexPlaylists() }));
   }
 
   async function onRemove(keys: string[]) {
@@ -431,7 +432,7 @@ export default function PlexPlaylistView({
     )
       return;
     await api.plexDeletePlaylist(playlistId);
-    qc.invalidateQueries({ queryKey: ["plex-playlists"] });
+    qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
     onBack();
   }