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

Entities/DTO/Class/ClassCreateDto.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO.Class
+{
+	public record ClassCreateDto
+	{
+		public CreateGradeDto Grade { get; init; }
+
+		[Required(ErrorMessage = "年级名称是必填项。")]
+		[StringLength(20, ErrorMessage = "年级名称不能超过 20 个字符。")]
+		public string ClassName { get; init; }
+
+		[Required(ErrorMessage = "年级级别是必填项。")]
+		public byte Index { get; init; }
+	}
+}