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.Domain/Entities/Track.cs

@@ -45,5 +45,7 @@ namespace Musify.Domain.Entities
         public ICollection<ListeningHistory> ListeningHistories { get; set; } = new List<ListeningHistory>();
 
         public ICollection<TrackTag> Tags { get; set; } = new List<TrackTag>();
+
+        public ICollection<TrackCoauthor> Coauthors { get; set; } = new List<TrackCoauthor>();
     }
 }