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/PlexSidebar.tsx

@@ -1,4 +1,5 @@
 import { useState, type ReactNode } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Layers, ListMusic, Plus, SlidersHorizontal, X } from "lucide-react";
@@ -84,7 +85,7 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
     setPlaylistOpen: onOpenPlaylist,
   } = usePlexActions();
   const collFade = useScrollFade();
-  const playlistsQ = useQuery({ queryKey: ["plex-playlists"], queryFn: () => api.plexPlaylists() });
+  const playlistsQ = useQuery({ queryKey: qk.plexPlaylists(), queryFn: () => api.plexPlaylists() });
   const [newPlaylist, setNewPlaylist] = useState("");
   const [editing, setEditing] = useState(false);
   const qc = useQueryClient();
@@ -93,12 +94,12 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
     if (!title) return;
     const pl = await api.plexCreatePlaylist(title);
     setNewPlaylist("");
-    qc.invalidateQueries({ queryKey: ["plex-playlists"] });
+    qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
     onOpenPlaylist(pl.id);
   }
   const { sortDir: _sd, collectionTitle: _ct, ...facetKey } = filters;
   const facetsQ = useQuery({
-    queryKey: ["plex-facets", scope, show, facetKey],
+    queryKey: qk.plexFacets(scope, show, facetKey),
     queryFn: () => api.plexFacets(scope, filters, show),
     enabled: !!scope,
     placeholderData: (prev) => prev,
@@ -108,7 +109,7 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
   const [collSearch, setCollSearch] = useState("");
   const collSearchDeb = useDebounced(collSearch.trim(), 300);
   const collectionsQ = useQuery({
-    queryKey: ["plex-collections", "union", collSearchDeb],
+    queryKey: qk.plexCollections("union", collSearchDeb),
     queryFn: () => api.plexCollections(undefined, collSearchDeb || undefined),
     enabled: !filters.collection,
   });