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/Tests/Musify.Application.Tests/YouTube/SearchYouTubeAlbumsQueryHandlerTests.cs

@@ -1,37 +0,0 @@
-using ErrorOr;
-using Musify.Application.Contracts;
-using Musify.Application.YouTube;
-using NSubstitute;
-using Xunit;
-
-namespace Musify.Application.Tests.YouTube
-{
-    public sealed class SearchYouTubeAlbumsQueryHandlerTests
-    {
-        private readonly IYouTubeMusicService youTubeMusicService = Substitute.For<IYouTubeMusicService>();
-
-        private SearchYouTubeAlbumsQueryHandler CreateHandler() => new(youTubeMusicService);
-
-        [Fact]
-        public async Task Handle_EmptyQuery_ReturnsValidationErrorWithoutCallingTheService()
-        {
-            var result = await CreateHandler().Handle(new SearchYouTubeAlbumsQuery(string.Empty, string.Empty), TestContext.Current.CancellationToken);
-
-            Assert.Equal(ErrorType.Validation, result.FirstError.Type);
-            await youTubeMusicService.DidNotReceiveWithAnyArgs()
-                .SearchAlbumsAsync(default!, default!, TestContext.Current.CancellationToken);
-        }
-
-        [Fact]
-        public async Task Handle_NonEmptyQuery_DelegatesToTheService()
-        {
-            var expected = new YouTubeAlbumSearchResult([], "next-token");
-            youTubeMusicService.SearchAlbumsAsync("query", "token", Arg.Any<CancellationToken>()).Returns(expected);
-
-            var result = await CreateHandler().Handle(new SearchYouTubeAlbumsQuery("query", "token"), TestContext.Current.CancellationToken);
-
-            Assert.False(result.IsError);
-            Assert.Same(expected, result.Value);
-        }
-    }
-}