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

Entities/DTO/Lesson/LessonResponseDto.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace Entities.DTO
+{
+    public class LessonResponseDto
+    {
+        public Guid Id { get; set; }
+        public string Title { get; set; } = string.Empty;
+        public string Description { get; set; } = string.Empty;
+        public Guid TextbookID { get; set; }
+        public DateTime CreatedAt { get; set; }
+        public DateTime? UpdatedAt { get; set; }
+    }
+}