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/api/tracks/+server.ts

@@ -7,9 +7,12 @@ export const GET: RequestHandler = async ({ url, locals, fetch }) => {
 	const name = url.searchParams.get('name')?.trim() || undefined;
 	const pageNumber = Math.max(1, Number(url.searchParams.get('pageNumber')) || 1);
 	const pageSize = Math.max(1, Number(url.searchParams.get('pageSize')) || EXPLORE_PAGE_SIZE);
+	const youtubeContinuationToken = url.searchParams.get('youtubeContinuationToken') || undefined;
 
 	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
-	const result = await api.GET('/tracks', { params: { query: { name, pageNumber, pageSize } } });
+	const result = await api.GET('/tracks', {
+		params: { query: { name, pageNumber, pageSize, youtubeContinuationToken } }
+	});
 
 	return json(unwrapOrError(result, 'No se pudieron cargar las canciones.'));
 };