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/AddToPlaylist.tsx

@@ -2,6 +2,7 @@ import { useEffect, useLayoutEffect, useRef, useState } from "react";
 import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { Check, ListPlus, Plus, Youtube } from "lucide-react";
 import { api, type Playlist } from "../lib/api";
 import { useDismiss } from "../lib/useDismiss";
@@ -28,7 +29,7 @@ export default function AddToPlaylist({
   const [busy, setBusy] = useState(false);
 
   const membership = useQuery({
-    queryKey: ["playlist-membership", videoId],
+    queryKey: qk.playlistMembership(videoId),
     queryFn: () => api.playlists(videoId),
     enabled: open,
   });
@@ -75,11 +76,11 @@ export default function AddToPlaylist({
   }, [open]);
 
   function refresh() {
-    qc.invalidateQueries({ queryKey: ["playlist-membership", videoId] });
-    qc.invalidateQueries({ queryKey: ["playlists"] });
+    qc.invalidateQueries({ queryKey: qk.playlistMembership(videoId) });
+    qc.invalidateQueries({ queryKey: qk.playlists() });
     // Also invalidate every open/cached playlist detail (["playlist", id]) so the
     // Playlists page reflects the add/remove when navigated to, not only after F5.
-    qc.invalidateQueries({ queryKey: ["playlist"] });
+    qc.invalidateQueries({ queryKey: qk.playlist() });
   }
 
   async function toggle(pl: Playlist) {