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/PlayerModal.tsx

@@ -3,6 +3,7 @@ import { useTranslation } from "react-i18next";
 import { createPortal } from "react-dom";
 import { useNavigationActions } from "./NavigationProvider";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import {
   AlertTriangle,
   ArrowLeft,
@@ -479,7 +480,7 @@ export default function PlayerModal({
     closeTimer.current = window.setTimeout(() => setShowDesc(false), 150);
   };
   const detail = useQuery({
-    queryKey: ["video-detail", currentVideoId],
+    queryKey: qk.videoDetail(currentVideoId),
     queryFn: () => api.videoDetail(currentVideoId),
     // On hover (for the description) and eagerly when navigated, so we can link the
     // linked video's channel. Both share the one cached result.
@@ -767,8 +768,8 @@ export default function PlayerModal({
       window.removeEventListener("pagehide", onHide);
       // Final flush, then refresh the feed + playlists so resume bars reflect this session.
       Promise.resolve(persist()).finally(() => {
-        qc.invalidateQueries({ queryKey: ["feed"] });
-        qc.invalidateQueries({ queryKey: ["playlist"] });
+        qc.invalidateQueries({ queryKey: qk.feed() });
+        qc.invalidateQueries({ queryKey: qk.playlist() });
       });
       const p = playerRef.current;
       if (p && typeof p.destroy === "function") {