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/ConfigPanel.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useMemo, useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Plug, RotateCcw, Send } from "lucide-react";
@@ -34,7 +35,7 @@ const GROUP_ORDER = [
 export default function ConfigPanel() {
   const { t } = useTranslation();
   const qc = useQueryClient();
-  const q = useQuery({ queryKey: ["admin-config"], queryFn: api.adminConfig });
+  const q = useQuery({ queryKey: qk.adminConfig(), queryFn: api.adminConfig });
   const data = q.data;
 
   const items = useMemo(() => (data ? Object.values(data.groups).flat() : []), [data]);
@@ -98,7 +99,7 @@ export default function ConfigPanel() {
           await api.setConfig(key, it.type === "int" ? Number(raw) : raw);
         }
       }
-      await qc.invalidateQueries({ queryKey: ["admin-config"] });
+      await qc.invalidateQueries({ queryKey: qk.adminConfig() });
       setReseedToken((n) => n + 1); // re-seed the draft from the saved server state
       setSaveState("saved");
       setTimeout(() => setSaveState((s) => (s === "saved" ? "idle" : s)), 2500);