Combine track search with live YouTube Music results

GET /tracks now merges the local catalog page with a live YouTube Music
search (deduplicated against tracks already provisioned locally) whenever a
name filter is given, appending the YouTube hits after the local matches and
carrying a continuation token for further pages. Each item is tagged with its
source via the new TrackSearchItemResponse/TracksSearchResponse shapes,
wrapping the existing TrackApplicationResponse or a raw YouTubeSongResult so
neither type had to change.

GET /youtube/search is removed along with its now-unused handler; the
web-player's explore page, its /api/tracks proxy, and the YouTube filler
helper are updated to call the combined endpoint instead of hitting the two
separately.

web-player/src/routes/+page.server.ts

@@ -44,7 +44,7 @@ export const load: PageServerLoad = async ({ locals, url, fetch }) => {
 		recentlyPlayedPromise
 	]);
 	const playlistItems = playlists.data?.items ?? [];
-	const latestItems = latest.data?.items ?? [];
+	const latestItems = (latest.data?.items ?? []).flatMap((item) => (item.track ? [item.track] : []));
 
 	return {
 		greeting: pickGreeting(new Date().getHours()),