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

Entities/DTO/Lesson/CreateLessonDto.cs

@@ -0,0 +1,17 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Entities.DTO
+{
+    public class CreateLessonDto
+    {
+        [Required(ErrorMessage = "课程标题是必填项")]
+        [StringLength(255, ErrorMessage = "课程标题长度不能超过255个字符")]
+        public string Title { get; set; } = string.Empty;
+
+        [StringLength(1000, ErrorMessage = "课程描述长度不能超过1000个字符")]
+        public string Description { get; set; } = string.Empty;
+
+        [Required(ErrorMessage = "教材ID是必填项")]
+        public Guid TextbookID { get; set; }
+    }
+}