feat: db models

Models/Database/MissionCategory.cs

@@ -0,0 +1,9 @@
+namespace GamificationService.Models.Database;
+
+public class MissionCategory
+{
+    public long Id { get; set; }
+    public string Title { get; set; } = string.Empty;
+
+    public ICollection<Mission> Missions { get; set; } = new List<Mission>();
+}