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/ValueObjects/CoauthorStatus.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum CoauthorStatus
+    {
+        Pending = 0,
+        Accepted = 1,
+        Rejected = 2
+    }
+}