fix: rev db model, added missing entities and helper functions

WebApp/Entities/EventRegistration.cs

@@ -3,7 +3,7 @@
     public class EventRegistration
     {
         public int EventId { get; set; }
-        public required string UserId { get; set; }
+        public required int VolunteerId { get; set; }
         public DateTime RegisteredAt { get; set; } = DateTime.UtcNow;
         public Event? Event { get; set; }
         public User? User { get; set; }