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/ChatThread.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useRef, useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
 import { Send } from "lucide-react";
@@ -35,7 +36,7 @@ export default function ChatThread({
   const needsKey = !isSystem && !ready;
 
   const q = useLiveQuery<{ partner: MessageUser; items: Message[] }>(
-    ["thread", partnerId],
+    qk.thread(partnerId),
     () => api.thread(partnerId),
     { intervalMs: POLL_MS, enabled: !needsKey },
   );
@@ -102,8 +103,8 @@ export default function ChatThread({
     const incoming = (q.data?.items ?? []).filter((m) => m.recipient_id === meId).length;
     if (incoming !== lastIncoming.current) {
       lastIncoming.current = incoming;
-      qc.invalidateQueries({ queryKey: ["conversations"] });
-      qc.invalidateQueries({ queryKey: ["message-unread"] });
+      qc.invalidateQueries({ queryKey: qk.conversations() });
+      qc.invalidateQueries({ queryKey: qk.messageUnread() });
     }
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [q.dataUpdatedAt, partnerId, meId, qc]);
@@ -123,8 +124,8 @@ export default function ChatThread({
     },
     onSuccess: () => {
       setDraft("");
-      qc.invalidateQueries({ queryKey: ["thread", partnerId] });
-      qc.invalidateQueries({ queryKey: ["conversations"] });
+      qc.invalidateQueries({ queryKey: qk.thread(partnerId) });
+      qc.invalidateQueries({ queryKey: qk.conversations() });
     },
     onError: (e) => {
       // The draft is kept (only cleared on success) so the user can retry. 400/409/422/500 already