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

Entities/Contracts/Textbook/KeyPoint.cs

@@ -18,14 +18,14 @@ namespace Entities.Contracts
 		public string Key { get; set; } = string.Empty;
 
 		
+
 		[Required]
 		public Guid LessonID { get; set; }
-
 		[ForeignKey(nameof(LessonID))]
-		public Lesson Lesson { get; set; }
+		public virtual Lesson Lesson { get; set; }
 
 
-		public ICollection<Question> Questions { get; set; }
+		public virtual ICollection<Question> Questions { get; set; }
 
 		public KeyPoint()
 		{