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

Entities/DTO/Textbook/TextbookDto.cs

@@ -0,0 +1,28 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    /// <summary>
+    /// 教材数据传输对象
+    /// </summary>
+    public class TextbookDto
+    {
+        public Guid Id { get; set; }
+
+        [Required(ErrorMessage = "教材标题是必填项。")]
+        [StringLength(100, ErrorMessage = "教材标题不能超过 100 个字符。")]
+        public string Title { get; set; }
+
+        [Required(ErrorMessage = "年级是必选项。")]
+        public string Grade { get; set; }
+
+        [Required(ErrorMessage = "出版社是必选项。")]
+        public string Publisher { get; set; }
+
+        [Required(ErrorMessage = "学科领域是必选项。")]
+        public string SubjectArea { get; set; }
+
+        public string Description { get; set; }
+    }
+}