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

Entities/DTO/Class/ClassDto.cs

@@ -0,0 +1,25 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class ClassDto
+    {
+        public Guid Id { get; set; }
+
+        public Guid GradeId { get; set; }
+
+        public GradeDto Grade { get; set; }
+
+        [Required(ErrorMessage = "班级名称是必填项。")]
+        [StringLength(30, ErrorMessage = "班级名称不能超过 30 个字符。")]
+        public string ClassName { get; set; }
+
+        public byte Index { get; set; }
+
+        public Guid? HeadTeacherId { get; set; }
+
+        public DateTime CreatedAt { get; set; }
+        public DateTime UpdatedAt { get; set; }
+    }
+}