refactor(fe): query-key factory + migrate feed/channels/playlists (R7 S2a)

Introduce lib/queryKeys.ts as the single source of truth for React Query
keys — the R7 driver was ~200 hand-written key literals across 51 files
(`["feed"]` alone in 15+), where one typo silently orphans a key so an
invalidate never matches.

S2a migrates the three highest-traffic domains (feed/video, channels,
playlists) plus the shared me/my-status/tags roots: ~90 literal sites in
18 files now call qk.*(). Pure substitution — each factory fn returns the
byte-identical array the call sites used before, so TanStack's prefix
matching is unchanged (qk.feed() still invalidates qk.feed(filters)).
Co-invalidation clusters are deliberately NOT bundled into helpers (they
differ per site), so no query's refetch scope changes.

Nullable keys (playlist(selectedId), ytSearch(q)) accept null so a null
segment is preserved (["playlist", null]) while a no-arg call is the bare
prefix key — guard is `!== undefined`.

Gate green: typecheck, eslint (0 err), prettier, 68 vitest. Smoke: app
boots, feed/facets/tags/my-status all 200, no console errors. Factory
extended to plex/messaging/admin/downloads/notifications in S2b.

frontend/src/components/Stats.tsx

@@ -1,6 +1,7 @@
 import { useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { History, Pause, Play, RefreshCw } from "lucide-react";
 import { api, type AdminQuotaRow, type Me } from "../lib/api";
 import { formatEta, quotaActionLabel } from "../lib/format";
@@ -55,14 +56,14 @@ export default function Stats() {
 function Overview({ me }: { me: Me }) {
   const { t } = useTranslation();
   const qc = useQueryClient();
-  const status = useQuery({ queryKey: ["my-status"], queryFn: api.myStatus });
+  const status = useQuery({ queryKey: qk.myStatus(), queryFn: api.myStatus });
   const usage = useQuery({ queryKey: ["my-usage"], queryFn: api.myUsage });
   const s = status.data;
   const syncSubs = useMutation({
     mutationFn: () => api.syncSubscriptions(),
     onSuccess: (r: { subscriptions?: number }) => {
-      qc.invalidateQueries({ queryKey: ["my-status"] });
-      qc.invalidateQueries({ queryKey: ["channels"] });
+      qc.invalidateQueries({ queryKey: qk.myStatus() });
+      qc.invalidateQueries({ queryKey: qk.channels() });
       notify({
         level: "success",
         message: t("stats.sync.synced", { count: r.subscriptions ?? 0 }),
@@ -73,8 +74,8 @@ function Overview({ me }: { me: Me }) {
   const deepAll = useMutation({
     mutationFn: () => api.deepAll(true),
     onSuccess: (r: { updated?: number }) => {
-      qc.invalidateQueries({ queryKey: ["my-status"] });
-      qc.invalidateQueries({ queryKey: ["channels"] });
+      qc.invalidateQueries({ queryKey: qk.myStatus() });
+      qc.invalidateQueries({ queryKey: qk.channels() });
       notify({
         level: "success",
         message: t("stats.sync.fullHistoryRequested", { count: r.updated ?? 0 }),
@@ -200,10 +201,10 @@ function SystemStats() {
   const qc = useQueryClient();
   const [days, setDays] = useState<number>(30);
   const q = useQuery({ queryKey: ["admin-quota", days], queryFn: () => api.adminQuota(days) });
-  const status = useQuery({ queryKey: ["my-status"], queryFn: api.myStatus });
+  const status = useQuery({ queryKey: qk.myStatus(), queryFn: api.myStatus });
   const pauseResume = useMutation({
     mutationFn: (paused: boolean) => (paused ? api.resumeSync() : api.pauseSync()),
-    onSuccess: () => qc.invalidateQueries({ queryKey: ["my-status"] }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: qk.myStatus() }),
   });
   const data = q.data;
   const maxDaily = Math.max(1, ...(data?.daily ?? []).map((d) => d.total));