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/Contracts/IDatabase.cs

@@ -32,6 +32,8 @@ namespace Musify.Application.Contracts
 
         public DbSet<UserFollow> UserFollows { get; }
 
+        public DbSet<TrackCoauthor> TrackCoauthors { get; }
+
         public Task<IDatabaseTransaction> BeginTransactionAsync(System.Data.IsolationLevel isolationLevel, CancellationToken cancellationToken);
 
         public DbSet<TEntity> Set<TEntity>() where TEntity : class;