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/AuditLog.tsx

@@ -1,4 +1,5 @@
 import { useMemo } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Trash2 } from "lucide-react";
@@ -18,7 +19,7 @@ export default function AuditLog() {
   const { t } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();
-  const q = useQuery({ queryKey: ["admin-audit"], queryFn: () => api.adminAudit() });
+  const q = useQuery({ queryKey: qk.adminAudit(), queryFn: () => api.adminAudit() });
   const data = q.data;
   const rows = useMemo(() => data?.items ?? [], [data]);
 
@@ -31,7 +32,7 @@ export default function AuditLog() {
   const clear = useMutation({
     mutationFn: () => api.clearAudit(),
     onSuccess: (r) => {
-      qc.invalidateQueries({ queryKey: ["admin-audit"] });
+      qc.invalidateQueries({ queryKey: qk.adminAudit() });
       notify({ level: "success", message: t("audit.cleared", { count: r.cleared }) });
     },
     onError: () => notify({ level: "error", message: t("audit.clearFailed") }),