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

Entities/DTO/Subject/SubjectDto.cs

@@ -0,0 +1,16 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class SubjectDto
+    {
+        public Guid Id { get; set; }
+
+        [Required(ErrorMessage = "科目名称是必填项。")]
+        [StringLength(20, ErrorMessage = "科目名称不能超过 20 个字符。")]
+        public string Name { get; set; }
+
+        public string Description { get; set; }
+    }
+}