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/ProfileEditor.tsx

@@ -1,4 +1,5 @@
 import { useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Lock, Pencil, Plus, Trash2 } from "lucide-react";
@@ -42,7 +43,7 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();
-  const profilesQ = useQuery({ queryKey: ["download-profiles"], queryFn: api.downloadProfiles });
+  const profilesQ = useQuery({ queryKey: qk.downloadProfiles(), queryFn: api.downloadProfiles });
   const profiles = profilesQ.data ?? [];
   const builtins = profiles.filter((p) => p.is_builtin);
   const mine = profiles.filter((p) => !p.is_builtin);
@@ -52,7 +53,7 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
   const [spec, setSpec] = useState<DownloadSpec>(BLANK);
   const [formOpen, setFormOpen] = useState(false);
 
-  const invalidate = () => qc.invalidateQueries({ queryKey: ["download-profiles"] });
+  const invalidate = () => qc.invalidateQueries({ queryKey: qk.downloadProfiles() });
   const startNew = () => {
     setEditing(null);
     setName("");