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/DownloadCenter.tsx

@@ -1,4 +1,5 @@
 import { lazy, Suspense, useEffect, useMemo, useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import {
@@ -281,7 +282,7 @@ function MetaEditModal({ job, onClose }: { job: DownloadJob; onClose: () => void
         extra_links: links.map((l) => l.trim()).filter(Boolean),
       }),
     onSuccess: () => {
-      qc.invalidateQueries({ queryKey: ["downloads"] });
+      qc.invalidateQueries({ queryKey: qk.downloads() });
       onClose();
     },
   });
@@ -365,7 +366,7 @@ function UsageBar() {
   // Poll live (like the library list) so the footprint updates the moment the worker finishes an
   // edit/download — enqueue only creates a queued job (0 bytes), the size lands asynchronously, so
   // a one-shot fetch at enqueue time would show a stale 0 B until a manual reload.
-  const usage = useLiveQuery(["download-usage"], api.downloadUsage, { intervalMs: 2000 });
+  const usage = useLiveQuery(qk.downloadUsage(), api.downloadUsage, { intervalMs: 2000 });
   const u = usage.data;
   if (!u) return null;
   const pct =
@@ -408,7 +409,7 @@ function QuotaModal({
   const { t } = useTranslation();
   const qc = useQueryClient();
   const q = useQuery({
-    queryKey: ["dl-quota", userId],
+    queryKey: qk.dlQuota(userId),
     queryFn: () => api.adminGetDownloadQuota(userId),
   });
   const [form, setForm] = useState<{
@@ -429,8 +430,8 @@ function QuotaModal({
         }
       : null);
   const done = () => {
-    qc.invalidateQueries({ queryKey: ["dl-quota", userId] });
-    qc.invalidateQueries({ queryKey: ["admin-storage"] });
+    qc.invalidateQueries({ queryKey: qk.dlQuota(userId) });
+    qc.invalidateQueries({ queryKey: qk.adminStorage() });
     onClose();
   };
   const save = useMutation({
@@ -516,7 +517,7 @@ function QuotaUserPicker({ onPick }: { onPick: (u: { id: number; email: string }
   const { t } = useTranslation();
   const [q, setQ] = useState("");
   const [open, setOpen] = useState(false);
-  const users = useQuery({ queryKey: ["admin-users"], queryFn: api.adminUsers });
+  const users = useQuery({ queryKey: qk.adminUsers(), queryFn: api.adminUsers });
   const list = (users.data ?? []).filter((u) => !u.is_demo);
   const filtered = useMemo(() => {
     const s = q.trim().toLowerCase();
@@ -565,8 +566,8 @@ function QuotaUserPicker({ onPick }: { onPick: (u: { id: number; email: string }
 
 function AdminSystem() {
   const { t } = useTranslation();
-  const storage = useLiveQuery(["admin-storage"], api.adminDownloadStorage, { intervalMs: 5000 });
-  const jobs = useLiveQuery(["admin-downloads"], api.adminDownloads, { intervalMs: 5000 });
+  const storage = useLiveQuery(qk.adminStorage(), api.adminDownloadStorage, { intervalMs: 5000 });
+  const jobs = useLiveQuery(qk.adminDownloads(), api.adminDownloads, { intervalMs: 5000 });
   const [quotaFor, setQuotaFor] = useState<{ id: number; email: string } | null>(null);
   const s = storage.data;
   return (
@@ -662,9 +663,9 @@ export default function DownloadCenter() {
   const [shareJob, setShareJob] = useState<DownloadJob | null>(null);
   const [editJob, setEditJob] = useState<DownloadJob | null>(null);
 
-  const jobsQ = useLiveQuery(["downloads"], api.downloads, { intervalMs: 2000 });
+  const jobsQ = useLiveQuery(qk.downloads(), api.downloads, { intervalMs: 2000 });
   const sharedQ = useQuery({
-    queryKey: ["downloads-shared"],
+    queryKey: qk.downloadsShared(),
     queryFn: api.downloadsShared,
     enabled: tab === "shared",
   });
@@ -698,7 +699,7 @@ export default function DownloadCenter() {
   // Remove a shared-with-me item from your list (only your grant; the owner's file is untouched).
   const removeShared = useMutation({
     mutationFn: (id: number) => api.removeSharedDownload(id),
-    onSuccess: () => qc.invalidateQueries({ queryKey: ["downloads-shared"] }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: qk.downloadsShared() }),
   });
 
   const confirmThen = async (title: string, body: string, fn: () => void) => {