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/ChatDock.tsx

@@ -1,4 +1,5 @@
 import { useEffect } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQueryClient } from "@tanstack/react-query";
 import { ChevronDown, ChevronUp, X } from "lucide-react";
@@ -34,10 +35,10 @@ export default function ChatDock({ meId }: { meId: number }) {
   useEffect(
     () =>
       onMessage(({ message: m, from }) => {
-        qc.invalidateQueries({ queryKey: ["conversations"] });
-        qc.invalidateQueries({ queryKey: ["message-unread"] });
+        qc.invalidateQueries({ queryKey: qk.conversations() });
+        qc.invalidateQueries({ queryKey: qk.messageUnread() });
         const partnerId = m.sender_id === meId ? m.recipient_id : m.sender_id;
-        qc.invalidateQueries({ queryKey: ["thread", partnerId] });
+        qc.invalidateQueries({ queryKey: qk.thread(partnerId) });
         // Auto-open/flash only for messages from someone else (not my own echo) and only for
         // real user conversations.
         if (m.kind === "user" && m.sender_id !== meId && from) {
@@ -51,8 +52,8 @@ export default function ChatDock({ meId }: { meId: number }) {
   useEffect(
     () =>
       onUnread(() => {
-        qc.invalidateQueries({ queryKey: ["message-unread"] });
-        qc.invalidateQueries({ queryKey: ["conversations"] });
+        qc.invalidateQueries({ queryKey: qk.messageUnread() });
+        qc.invalidateQueries({ queryKey: qk.conversations() });
       }),
     [qc],
   );