Добавил домен

backend/UniVerse.Domain/Entities/UserAchievement.cs

@@ -0,0 +1,13 @@
+namespace UniVerse.Domain.Entities;
+
+public class UserAchievement
+{
+    public int Id { get; set; }
+    public int UserId { get; set; }
+    public int AchievementId { get; set; }
+    public DateTime AwardedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public User User { get; set; } = null!;
+    public Achievement Achievement { get; set; } = null!;
+}