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/(app)/albums/external/[source]/[externalId]/+page.server.ts

@@ -10,13 +10,15 @@ export const load: PageServerLoad = async ({ params, locals, url, fetch }) => {
 	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
 
 	const [albumRes, playlistsRes] = await Promise.all([
-		api.GET('/youtube/albums/{albumId}', { params: { path: { albumId: params.albumId } } }),
+		api.GET('/albums/external/{source}/{externalId}', {
+			params: { path: { source: params.source, externalId: params.externalId } }
+		}),
 		api.GET('/playlists/users/{userId}', {
 			params: { path: { userId: user.sub }, query: { pageNumber: 1, pageSize: 50 } }
 		})
 	]);
 
-	const album = unwrapOrError(albumRes, 'No se pudo cargar el álbum de YouTube Music.', 404);
+	const album = unwrapOrError(albumRes, 'No se pudo cargar el álbum.', 404);
 
 	return { album, playlists: playlistsRes.data?.items ?? [], section: null };
 };