Generalize YouTube-backed endpoints behind a source parameter

Removes the last YouTube-specific routes (/youtube/*) and merges them
into the general track/album/playlist endpoints, keyed by a `source`
string validated against TrackSource. This mirrors the earlier track
search merge for albums and playlist-track additions:

- GetAlbums now merges local albums with live YouTube Music search
  results, paginated via a youtubeContinuationToken, deduped against
  already-materialized ExternalAlbum rows.
- New GET /albums/external/{source}/{externalId} and
  GET /tracks/external/{source}/{externalId}/stream replace the
  YouTube-only equivalents.
- POST /playlists/{id}/tracks replaces the separate
  .../youtube-tracks endpoint, accepting an optional Source/ExternalId
  pair alongside TrackId.
- AlbumSearchItemResponse/AlbumsSearchResponse follow the same
  discriminated-union + trailing-HasNextPage-field pattern already
  used for track search, so HasNextPage stays a required field in the
  generated OpenAPI schema.
- Frontend routes renamed accordingly (/albums/youtube/[albumId] ->
  /albums/external/[source]/[externalId], etc.) with a single
  /api/thumbnail proxy replacing /api/youtube/thumbnail.

Musify.Api.Endpoints.YouTubeEndpoints and the standalone
SearchYouTubeAlbums query are removed as a result.

web-player/src/routes/api/tracks/external/[source]/[externalId]/stream/+server.ts

@@ -0,0 +1,14 @@
+import type { RequestHandler } from './$types';
+import { json } from '@sveltejs/kit';
+import { createApiClient, requireAccessToken, unwrapOrError } from '$lib/server/api';
+
+export const GET: RequestHandler = async ({ params, locals, fetch }) => {
+	const accessToken = requireAccessToken(locals, 'Inicia sesión para reproducir.');
+
+	const api = createApiClient({ fetch, accessToken });
+	const result = await api.GET('/tracks/external/{source}/{externalId}/stream', {
+		params: { path: { source: params.source, externalId: params.externalId } }
+	});
+
+	return json(unwrapOrError(result, 'No se pudo obtener el stream.'));
+};