重构项目结构，移除Assignment相关功能，优化Submission模块

Entities/DTO/Class/RegisterUserToClassDto.cs

@@ -0,0 +1,18 @@
+using Entities.Contracts;
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class RegisterUserToClassDto
+    {
+
+        public Guid UserId { get; set; }
+
+        public Guid ClassId { get; set; }
+
+        public UserRoles Role { get; set; } = UserRoles.Student; 
+
+        public Guid? SubjectArea { get; set; }
+    }
+}