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/ShareDialog.tsx

@@ -1,4 +1,5 @@
 import { useMemo, useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Check, Copy, Link2, Lock, Trash2, UserPlus, Eye } from "lucide-react";
@@ -17,7 +18,7 @@ function UserShare({ job }: { job: DownloadJob }) {
   const { t } = useTranslation();
   const [q, setQ] = useState("");
   const [open, setOpen] = useState(false);
-  const recipients = useQuery({ queryKey: ["share-recipients"], queryFn: api.shareRecipients });
+  const recipients = useQuery({ queryKey: qk.shareRecipients(), queryFn: api.shareRecipients });
   const list = recipients.data ?? [];
   const filtered = useMemo(() => {
     const s = q.trim().toLowerCase();
@@ -245,14 +246,14 @@ function LinkShare({ job }: { job: DownloadJob }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
   const links = useQuery({
-    queryKey: ["download-links", job.id],
+    queryKey: qk.downloadLinks(job.id),
     queryFn: () => api.downloadLinks(job.id),
   });
   const [creating, setCreating] = useState(false);
   const [allowDownload, setAllowDownload] = useState(false);
   const [expiryDays, setExpiryDays] = useState<number | null>(null);
   const [password, setPassword] = useState("");
-  const invalidate = () => qc.invalidateQueries({ queryKey: ["download-links", job.id] });
+  const invalidate = () => qc.invalidateQueries({ queryKey: qk.downloadLinks(job.id) });
 
   const create = useMutation({
     mutationFn: () =>