Добавил домен

backend/UniVerse.Domain/Entities/Achievement.cs

@@ -0,0 +1,16 @@
+namespace UniVerse.Domain.Entities;
+
+public class Achievement
+{
+    public int Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public string? Description { get; set; }
+    public string? IconUrl { get; set; }
+    public int XpReward { get; set; }
+    public int CoinReward { get; set; }
+    public string? Condition { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public ICollection<UserAchievement> UserAchievements { get; set; } = new List<UserAchievement>();
+}