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/albums/external/[source]/[externalId]/tracks/+server.ts

@@ -4,9 +4,9 @@ import { createApiClient, unwrapOrError } from '$lib/server/api';
 
 export const GET: RequestHandler = async ({ params, locals, fetch }) => {
 	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
-	const result = await api.GET('/youtube/albums/{albumId}', {
-		params: { path: { albumId: params.albumId } }
+	const result = await api.GET('/albums/external/{source}/{externalId}', {
+		params: { path: { source: params.source, externalId: params.externalId } }
 	});
 
-	return json(unwrapOrError(result, 'No se pudo cargar el álbum de YouTube.'));
+	return json(unwrapOrError(result, 'No se pudo cargar el álbum.'));
 };

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

@@ -6,9 +6,9 @@ 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('/youtube/tracks/{videoId}/stream', {
-		params: { path: { videoId: params.videoId } }
+	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 de YouTube.'));
+	return json(unwrapOrError(result, 'No se pudo obtener el stream.'));
 };