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/SyncStatus.tsx

@@ -1,6 +1,7 @@
 import { useRef, useState } from "react";
 import { createPortal } from "react-dom";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { CheckCircle2, Clock, Database, History, Loader2, Pause, Play } from "lucide-react";
 import { api, type MyStatus } from "../lib/api";
@@ -27,7 +28,7 @@ export default function SyncStatus({
   const [pos, setPos] = useState<{ top: number; right: number }>({ top: 0, right: 0 });
   useDismiss(open, () => setOpen(false), [chipRef, popRef]);
   const { data } = useQuery({
-    queryKey: ["my-status"],
+    queryKey: qk.myStatus(),
     queryFn: api.myStatus,
     // Track the scheduler near-real-time: poll even when the tab is backgrounded, and
     // refresh immediately on tab focus (the global default disables focus refetch), so the
@@ -47,7 +48,7 @@ export default function SyncStatus({
 
   const toggle = useMutation({
     mutationFn: () => (data?.paused ? api.resumeSync() : api.pauseSync()),
-    onSuccess: () => qc.invalidateQueries({ queryKey: ["my-status"] }),
+    onSuccess: () => qc.invalidateQueries({ queryKey: qk.myStatus() }),
   });
 
   if (!data) return null;