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/PlexInfo.tsx

@@ -1,4 +1,5 @@
 import { useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQueryClient } from "@tanstack/react-query";
 import { Check, ExternalLink, FolderPlus, ListPlus, Play, RotateCcw, Star, X } from "lucide-react";
@@ -52,7 +53,7 @@ export default function PlexInfo({
   // artBg/showCast + their toggles and the raw savePref persister. The collection-strip source hiding
   // is PlexInfo-specific state, persisted through the same savePref.
   const { artBg, showCast, toggleArtBg, toggleCast, savePref } = useDetailPrefs();
-  const prefs = (qc.getQueryData<{ preferences?: Record<string, unknown> }>(["me"])?.preferences ??
+  const prefs = (qc.getQueryData<{ preferences?: Record<string, unknown> }>(qk.me())?.preferences ??
     {}) as { plexHiddenStripSources?: string[] };
   // Which collection-strip source buckets the user has HIDDEN (default: none → all shown). Toggled
   // per type in the customize menu (Collections / IMDb / TMDb / …), only for sources present here.
@@ -66,7 +67,7 @@ export default function PlexInfo({
       return next;
     });
   };
-  const isAdmin = qc.getQueryData<{ role?: string }>(["me"])?.role === "admin";
+  const isAdmin = qc.getQueryData<{ role?: string }>(qk.me())?.role === "admin";
   const [editorOpen, setEditorOpen] = useState(false);
   const [playlistOpen, setPlaylistOpen] = useState(false);
 
@@ -77,9 +78,9 @@ export default function PlexInfo({
   const inProgress = (detail.position_seconds ?? 0) > 0 && detail.status !== "watched";
   const setState = async (status: "new" | "watched") => {
     await api.plexSetState(detail.id, status).catch(() => {});
-    qc.invalidateQueries({ queryKey: ["plex-item", detail.id] });
-    qc.invalidateQueries({ queryKey: ["plex-library"] });
-    qc.invalidateQueries({ queryKey: ["plex-show"] });
+    qc.invalidateQueries({ queryKey: qk.plexItem(detail.id) });
+    qc.invalidateQueries({ queryKey: qk.plexLibrary() });
+    qc.invalidateQueries({ queryKey: qk.plexShow() });
     onStateChange?.();
   };