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/Scheduler.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useState } from "react";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import i18n from "../i18n";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
@@ -340,7 +341,7 @@ export default function Scheduler() {
   const confirm = useConfirm();
   // Poll faster while any job is running so live progress is smooth, then ease back when
   // idle. Derived from the freshest data by react-query itself (see useLiveQuery).
-  const q = useLiveQuery<SchedulerStatus>(["scheduler"], api.schedulerStatus, {
+  const q = useLiveQuery<SchedulerStatus>(qk.scheduler(), api.schedulerStatus, {
     intervalMs: (d) => (d?.jobs?.some((j) => j.running) ? FAST_POLL_MS : POLL_MS),
   });
   const data = q.data;
@@ -376,14 +377,14 @@ export default function Scheduler() {
 
   const pauseResume = useMutation({
     mutationFn: (paused: boolean) => (paused ? api.resumeSync() : api.pauseSync()),
-    onSuccess: () => qc.invalidateQueries({ queryKey: ["scheduler"] }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: qk.scheduler() }),
     onError: () => notify({ level: "error", message: t("scheduler.toggleFailed") }),
   });
 
   const intervalMut = useMutation({
     mutationFn: (v: { jobId: string; minutes: number }) =>
       api.updateSchedulerJob(v.jobId, v.minutes),
-    onSuccess: () => qc.invalidateQueries({ queryKey: ["scheduler"] }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: qk.scheduler() }),
     onError: () => notify({ level: "error", message: t("scheduler.intervalFailed") }),
   });
 
@@ -396,7 +397,7 @@ export default function Scheduler() {
         level: "success",
         message: t("scheduler.triggered", { job: t(`scheduler.jobs.${jobId}`, jobId) }),
       });
-      qc.invalidateQueries({ queryKey: ["scheduler"] });
+      qc.invalidateQueries({ queryKey: qk.scheduler() });
     },
   });
   const runAllMut = useMutation({
@@ -406,12 +407,12 @@ export default function Scheduler() {
         level: "success",
         message: t("scheduler.triggeredAll", { count: res.started.length }),
       });
-      qc.invalidateQueries({ queryKey: ["scheduler"] });
+      qc.invalidateQueries({ queryKey: qk.scheduler() });
     },
   });
   const batchMut = useMutation({
     mutationFn: (v: number) => api.updateMaintenanceBatch(v),
-    onSuccess: () => qc.invalidateQueries({ queryKey: ["scheduler"] }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: qk.scheduler() }),
   });
   const purgeMut = useMutation({
     mutationFn: () => api.purgeDiscovery(),
@@ -419,7 +420,7 @@ export default function Scheduler() {
       const removed =
         (res.search_videos_deleted ?? 0) + (res.videos_deleted ?? 0) + (res.channels_deleted ?? 0);
       notify({ level: "success", message: t("scheduler.purgedDiscovery", { count: removed }) });
-      qc.invalidateQueries({ queryKey: ["scheduler"] });
+      qc.invalidateQueries({ queryKey: qk.scheduler() });
     },
   });