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.

backend/Core/Musify.Application/YouTube/SearchYouTubeAlbums.cs

@@ -1,21 +0,0 @@
-using ErrorOr;
-using Mediator;
-using Musify.Application.Contracts;
-
-namespace Musify.Application.YouTube
-{
-    public record SearchYouTubeAlbumsQuery(string Query, string ContinuationToken)
-        : IQuery<ErrorOr<YouTubeAlbumSearchResult>>;
-
-    public class SearchYouTubeAlbumsQueryHandler(IYouTubeMusicService youTubeMusicService)
-        : IQueryHandler<SearchYouTubeAlbumsQuery, ErrorOr<YouTubeAlbumSearchResult>>
-    {
-        public async ValueTask<ErrorOr<YouTubeAlbumSearchResult>> Handle(SearchYouTubeAlbumsQuery request, CancellationToken cancellationToken)
-        {
-            if (string.IsNullOrEmpty(request.Query))
-                return Error.Validation(description: "The search query must not be empty.");
-
-            return await youTubeMusicService.SearchAlbumsAsync(request.Query, request.ContinuationToken, cancellationToken);
-        }
-    }
-}