feat(backend): search albums on YouTube Music

Two read-only endpoints: the album search and the album detail with its
tracklist. Both go through the same single-flight cache as the song search,
and the detail one especially benefits since it costs two upstream calls, the
browse id lookup and the album itself, for metadata that never changes.
Paginator handling is now shared between the song and album searches.

backend/Musify.Api/Endpoints/YouTubeEndpoints.cs

@@ -22,6 +22,22 @@ public static class YouTubeEndpoints
             .Produces(StatusCodes.Status401Unauthorized)
             .ProducesValidationProblem();
 
+        group.MapGet("/albums", SearchAlbums)
+            .WithName("SearchYouTubeAlbums")
+            .WithSummary("Search Albums On YouTube Music.")
+            .RequireAuthorization()
+            .Produces<YouTubeAlbumSearchResult>()
+            .Produces(StatusCodes.Status401Unauthorized)
+            .ProducesValidationProblem();
+
+        group.MapGet("/albums/{albumId}", GetAlbum)
+            .WithName("GetYouTubeAlbum")
+            .WithSummary("Get A YouTube Music Album With Its Tracklist.")
+            .RequireAuthorization()
+            .Produces<YouTubeAlbumDetail>()
+            .Produces(StatusCodes.Status401Unauthorized)
+            .Produces(StatusCodes.Status404NotFound);
+
         group.MapGet("/tracks/{videoId}/stream", GetTrackStream)
             .WithName("GetYouTubeTrackStream")
             .WithSummary("Resolve Playback For A YouTube Track: Server Stream If Downloaded, Direct YouTube Stream Otherwise.")
@@ -46,6 +62,28 @@ public static class YouTubeEndpoints
         return result.ToHttpResult();
     }
 
+    private static async Task<IResult> SearchAlbums(
+        IMediator mediator,
+        CancellationToken cancellationToken,
+        string query,
+        string? continuation = null)
+    {
+        var result = await mediator.Send(
+            new SearchYouTubeAlbumsQuery(query, continuation ?? string.Empty),
+            cancellationToken);
+
+        return result.ToHttpResult();
+    }
+
+    private static async Task<IResult> GetAlbum(
+        IMediator mediator,
+        string albumId,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(new GetYouTubeAlbumQuery(albumId), cancellationToken);
+        return result.ToHttpResult();
+    }
+
     private static async Task<IResult> SearchTracks(
         IMediator mediator,
         CancellationToken cancellationToken,