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/PlaylistsRail.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useMemo, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
 import { ArrowDown, ArrowUp, ListVideo, Pin, Plus, RefreshCw, Youtube } from "lucide-react";
 import { api, type Playlist } from "../lib/api";
 import { formatDuration } from "../lib/format";
@@ -54,7 +55,7 @@ export default function PlaylistsRail({
   const scrolledRef = useRef(false);
 
   const listQuery = useQuery({
-    queryKey: ["playlists"],
+    queryKey: qk.playlists(),
     queryFn: () => api.playlists(),
     refetchOnWindowFocus: true,
   });
@@ -114,8 +115,8 @@ export default function PlaylistsRail({
     setSyncing(true);
     try {
       const r = await api.syncYoutubePlaylists();
-      qc.invalidateQueries({ queryKey: ["playlists"] });
-      qc.invalidateQueries({ queryKey: ["playlist"] });
+      qc.invalidateQueries({ queryKey: qk.playlists() });
+      qc.invalidateQueries({ queryKey: qk.playlist() });
       notify({ level: "success", message: t("playlists.syncedToast", { count: r.synced }) });
     } catch (e) {
       notifyYouTubeActionError(e, t("playlists.syncFailed"));
@@ -128,7 +129,7 @@ export default function PlaylistsRail({
     mutationFn: (name: string) => api.createPlaylist(name),
     onSuccess: (pl: Playlist) => {
       setNewName("");
-      qc.invalidateQueries({ queryKey: ["playlists"] });
+      qc.invalidateQueries({ queryKey: qk.playlists() });
       setSelectedId(pl.id);
     },
   });