feat(backend): save a YouTube album with all of its tracks

POST /albums/youtube/{albumId} creates the album in your library and links
every song in it, provisioning each one as an external track without queueing
any download. The external id is kept on the album so saving it twice returns
a conflict instead of a duplicate, and albums can now hold external tracks,
not just the ones you uploaded.

backend/Musify.Api/Endpoints/AlbumEndpoints.cs

@@ -50,6 +50,15 @@ public static class AlbumEndpoints
             .Produces(StatusCodes.Status401Unauthorized)
             .Produces(StatusCodes.Status404NotFound);
 
+        group.MapPost("/youtube/{albumId}", SaveYouTubeAlbum)
+            .WithName("SaveYouTubeAlbum")
+            .WithSummary("Save A YouTube Music Album To Your Library With All Of Its Tracks.")
+            .RequireAuthorization()
+            .Produces<AlbumApplicationResponse>(StatusCodes.Status201Created)
+            .Produces(StatusCodes.Status401Unauthorized)
+            .Produces(StatusCodes.Status404NotFound)
+            .Produces(StatusCodes.Status409Conflict);
+
         group.MapPut("/{albumId}", UpdateAlbum)
             .WithName("UpdateAlbum")
             .WithSummary("Update An Album.")
@@ -143,6 +152,19 @@ public static class AlbumEndpoints
         return result.ToCreatedResult(album => $"/albums/{album.Id}");
     }
 
+    private static async Task<IResult> SaveYouTubeAlbum(
+        IMediator mediator,
+        CurrentUser currentUser,
+        string albumId,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(
+            new SaveYouTubeAlbumCommand(currentUser.RequiredId, albumId),
+            cancellationToken);
+
+        return result.ToCreatedResult(album => $"/albums/{album.Id}");
+    }
+
     private static async Task<IResult> UpdateAlbum(
         IMediator mediator,
         CurrentUser currentUser,