Converted the project in C#, added Entity Framework Core.

User.cs

@@ -0,0 +1,14 @@
+namespace IsleBot;
+
+public class User
+{
+    public int Id { get; private set; }
+    public string UserName { get; set; }
+    public List<Card> Cards { get; set; }
+
+    public User(int id, string userName) {
+        Id = id;
+        UserName = userName;
+        Cards = new List<Card>();
+    }
+}