Join, leave, registrations endpoints

todo: remove smb from event endpoint

WebApp/DTOs/EventRegistrationDto.cs

@@ -0,0 +1,11 @@
+using System.ComponentModel.DataAnnotations;
+using WebApp.Entities;
+
+namespace WebApp.DTOs;
+
+// Output values in JSON file
+public record class EventRegistrationDto(
+    int EventId,
+    int UserId,
+    DateTime RegisteredAt
+    );