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/PlexPlayer.tsx

@@ -9,6 +9,7 @@ import {
   type ReactNode,
   type WheelEvent as ReactWheelEvent,
 } from "react";
+import { qk } from "../lib/queryKeys";
 import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
@@ -196,7 +197,7 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
   tRef.current = t;
   const qc = useQueryClient();
   const [id, setId] = useState(itemId);
-  const detailQ = useQuery({ queryKey: ["plex-item", id], queryFn: () => api.plexItem(id) });
+  const detailQ = useQuery({ queryKey: qk.plexItem(id), queryFn: () => api.plexItem(id) });
   const detail = detailQ.data;
   const duration = detail?.duration_seconds ?? 0;
 
@@ -513,12 +514,10 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
       // unmount — patching while mounted would retrigger the loadSession effect and reseek mid-play.
       if (absRef.current > 0 && durationRef.current > 0) {
         const pos = Math.floor(absRef.current);
-        qc.setQueryData(["plex-item", id], (old) =>
-          old ? { ...old, position_seconds: pos } : old,
-        );
+        qc.setQueryData(qk.plexItem(id), (old) => (old ? { ...old, position_seconds: pos } : old));
       }
-      qc.invalidateQueries({ queryKey: ["plex-library"] });
-      qc.invalidateQueries({ queryKey: ["plex-show"] });
+      qc.invalidateQueries({ queryKey: qk.plexLibrary() });
+      qc.invalidateQueries({ queryKey: qk.plexShow() });
     };
   }, [id, qc]);