Add track coauthors with invitations, acceptance and coauthor-aware track responses

Owners invite existing users to coauthor a track; invitees accept or reject,
and either side can remove a coauthor. Only accepted coauthors appear in track
responses and in the coauthor's public track list. The owner also receives the
full invitation list with statuses on the user tracks response.

backend/Core/Musify.Application/Tracks/Responses/TrackApplicationResponse.cs

@@ -15,7 +15,8 @@ namespace Musify.Application.Tracks.Responses
         long OwnerUserId,
         IReadOnlyCollection<Genre> Tags,
         bool IsExplicit,
-        bool IsPublished)
+        bool IsPublished,
+        IReadOnlyList<TrackCoauthorReference> Coauthors)
     {
         public static TrackApplicationResponse FromEntity(Track track, int listensCount) =>
             new(
@@ -30,6 +31,7 @@ namespace Musify.Application.Tracks.Responses
                 track.OwnerUserId,
                 track.Tags.Select(tag => tag.Tag).ToList(),
                 track.IsExplicit,
-                track.IsPublished);
+                track.IsPublished,
+                []);
     }
 }

backend/Core/Musify.Application/Tracks/Responses/TrackCoauthorResponses.cs

@@ -0,0 +1,8 @@
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Application.Tracks.Responses
+{
+    public record TrackCoauthorReference(long Id, string Name);
+
+    public record TrackCoauthorInviteReference(long UserId, string Name, CoauthorStatus Status);
+}

backend/Core/Musify.Application/Tracks/Responses/TrackProjections.cs

@@ -27,9 +27,15 @@ namespace Musify.Application.Tracks.Responses
                 track.OwnerUserId,
                 track.Tags.Select(tag => tag.Tag).ToList(),
                 track.IsExplicit,
-                track.IsPublished));
+                track.IsPublished,
+                track.Coauthors
+                    .Where(coauthor => coauthor.Status == CoauthorStatus.Accepted)
+                    .OrderBy(coauthor => coauthor.RespondedAt)
+                    .ThenBy(coauthor => coauthor.UserId)
+                    .Select(coauthor => new TrackCoauthorReference(coauthor.UserId, coauthor.User.Name))
+                    .ToList()));
 
-        public static IQueryable<UserTrackApplicationResponse> SelectUserResponse(this IQueryable<Track> query) =>
+        public static IQueryable<UserTrackApplicationResponse> SelectUserResponse(this IQueryable<Track> query, bool includeInvites = false) =>
             query.Select(track => new UserTrackApplicationResponse(
                 track.Id,
                 track.Title,
@@ -48,6 +54,19 @@ namespace Musify.Application.Tracks.Responses
                     .OrderBy(albumTrack => albumTrack.AddedAt)
                     .ThenBy(albumTrack => albumTrack.AlbumId)
                     .Select(albumTrack => new TrackAlbumReference(albumTrack.AlbumId, albumTrack.Album.Title))
-                    .ToList()));
+                    .ToList(),
+                track.Coauthors
+                    .Where(coauthor => coauthor.Status == CoauthorStatus.Accepted)
+                    .OrderBy(coauthor => coauthor.RespondedAt)
+                    .ThenBy(coauthor => coauthor.UserId)
+                    .Select(coauthor => new TrackCoauthorReference(coauthor.UserId, coauthor.User.Name))
+                    .ToList(),
+                includeInvites
+                    ? track.Coauthors
+                        .OrderBy(coauthor => coauthor.InvitedAt)
+                        .ThenBy(coauthor => coauthor.UserId)
+                        .Select(coauthor => new TrackCoauthorInviteReference(coauthor.UserId, coauthor.User.Name, coauthor.Status))
+                        .ToList()
+                    : new List<TrackCoauthorInviteReference>()));
     }
 }

backend/Core/Musify.Application/Tracks/Responses/UserTrackApplicationResponse.cs

@@ -17,5 +17,7 @@ namespace Musify.Application.Tracks.Responses
         IReadOnlyCollection<Genre> Tags,
         bool IsExplicit,
         bool IsPublished,
-        IReadOnlyList<TrackAlbumReference> Albums);
+        IReadOnlyList<TrackAlbumReference> Albums,
+        IReadOnlyList<TrackCoauthorReference> Coauthors,
+        IReadOnlyList<TrackCoauthorInviteReference> CoauthorInvites);
 }