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.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Core/Musify.Application/Tracks/GetTracks.cs

@@ -67,7 +67,7 @@ namespace Musify.Application.Tracks
                 (youtubeItems, nextContinuationToken, youtubeUnavailable) =
                     await SearchYouTubeAsync(request.Name, request.YoutubeContinuationToken, knownVideoIds, cancellationToken);
 
-            return new TracksSearchResponse(
+            return TracksSearchResponse.Create(
                 [.. localItems, .. youtubeItems],
                 pagedEntities.PageNumber,
                 pagedEntities.PageSize,