🐛 Fix the message pack serializer

DysonNetwork.Shared/Models/UnpaidAccount.cs

@@ -0,0 +1,12 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace DysonNetwork.Shared.Models
+{
+    public class SnUnpaidAccount
+    {
+        [Key]
+        public Guid AccountId { get; set; }
+        public DateTime MarkedAt { get; set; }
+    }
+}