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/PlayerModal.tsx

@@ -381,7 +381,7 @@ export default function PlayerModal({
   // so they apply to every player and survive reloads. A ref feeds the (once-bound) end handler.
   const savedPrefs = useMemo(
     () =>
-      (qc.getQueryData<{ preferences?: Record<string, unknown> }>(["me"])?.preferences ?? {}) as {
+      (qc.getQueryData<{ preferences?: Record<string, unknown> }>(qk.me())?.preferences ?? {}) as {
         playerAutoAdvance?: AutoMode;
         playerLoop?: LoopMode;
       },
@@ -393,7 +393,7 @@ export default function PlayerModal({
   modeRef.current = { autoMode, loopMode };
   const persistPref = (patch: Record<string, string>) => {
     api.savePrefs(patch).catch(() => {});
-    qc.setQueryData<{ preferences?: Record<string, unknown> } | undefined>(["me"], (m) =>
+    qc.setQueryData<{ preferences?: Record<string, unknown> } | undefined>(qk.me(), (m) =>
       m ? { ...m, preferences: { ...(m.preferences || {}), ...patch } } : m,
     );
   };