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/PrefsProvider.tsx

@@ -8,6 +8,7 @@ import {
   useState,
   type ReactNode,
 } from "react";
+import { qk } from "../lib/queryKeys";
 import { useQuery } from "@tanstack/react-query";
 import { api } from "../lib/api";
 import {
@@ -68,7 +69,7 @@ export function PrefsProvider({ children }: { children: ReactNode }) {
   const saveMsgTimer = useRef<number | undefined>(undefined);
   const saveTimer = useRef<number | undefined>(undefined);
   // Passive observer of the account App fetches (enabled:false → reads cache, re-renders on update).
-  const { data: me } = useQuery({ queryKey: ["me"], queryFn: api.me, enabled: false });
+  const { data: me } = useQuery({ queryKey: qk.me(), queryFn: api.me, enabled: false });
 
   // Live-apply the prefs locally for instant preview (their localStorage mirrors update via the
   // stores too); persistence to the server is the auto-save effect below.