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/SettingsPanel.tsx

@@ -1,4 +1,5 @@
 import { useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Bell, Check, Loader2, Monitor, RefreshCw, Trash2, User } from "lucide-react";
@@ -387,7 +388,7 @@ function SignInMethods({ me }: { me: Me }) {
       setCurrent("");
       setNext("");
       // Refresh `me` so has_password flips and the section switches to "Change password".
-      await qc.invalidateQueries({ queryKey: ["me"] });
+      await qc.invalidateQueries({ queryKey: qk.me() });
       notify({
         level: "success",
         message: t("settings.account.password.saved", { email: me.email }),
@@ -516,7 +517,7 @@ function SignInMethods({ me }: { me: Me }) {
 function PlexWatchSync() {
   const { t, i18n } = useTranslation();
   const qc = useQueryClient();
-  const link = useQuery({ queryKey: ["plex-watch-link"], queryFn: () => api.plexWatchLink() });
+  const link = useQuery({ queryKey: qk.plexWatchLink(), queryFn: () => api.plexWatchLink() });
   const enabled = link.data?.sync_enabled ?? false;
 
   const announce = (imp: { watched: number; in_progress: number } | null | undefined) => {
@@ -529,7 +530,7 @@ function PlexWatchSync() {
         }),
       });
       // The browse grid's watch badges read plex_states — refresh them.
-      qc.invalidateQueries({ queryKey: ["plex"] });
+      qc.invalidateQueries({ queryKey: qk.plex() });
     }
   };
 
@@ -539,14 +540,14 @@ function PlexWatchSync() {
   const toggle = useMutation({
     mutationFn: (next: boolean) => api.plexWatchSetLink(next),
     onSuccess: (res) => {
-      qc.setQueryData(["plex-watch-link"], res);
+      qc.setQueryData(qk.plexWatchLink(), res);
       announce(res.import);
     },
   });
   const reimport = useMutation({
     mutationFn: () => api.plexWatchImport(),
     onSuccess: (res) => {
-      qc.setQueryData(["plex-watch-link"], res);
+      qc.setQueryData(qk.plexWatchLink(), res);
       announce(res.import);
     },
   });