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/lib/player/player.svelte.ts

@@ -305,7 +305,7 @@ class PlayerState {
 	}
 
 	async #resolveYouTubeSrc(videoId: string): Promise<string> {
-		const res = await fetch(`/api/youtube/tracks/${videoId}/stream`);
+		const res = await fetch(`/api/tracks/external/youtube/${videoId}/stream`);
 		if (!res.ok) throw new Error(await readErrorMessage(res));
 		const { mode, streamUrl, ticket } = (await res.json()) as {
 			mode: string;