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

Entities/Contracts/QuestionContext.cs

@@ -13,13 +13,13 @@ namespace Entities.Contracts
 
 		public string Description { get; set; } = string.Empty;
 
-		[InverseProperty(nameof(AssignmentQuestion.QuestionContext))]	
-		public ICollection<AssignmentQuestion>? Questions { get; set; } = new List<AssignmentQuestion>();
+		[InverseProperty(nameof(ExamQuestion.QuestionContext))]	
+		public virtual ICollection<ExamQuestion>? Questions { get; set; } = new List<ExamQuestion>();
 
 
 		public QuestionContext()
 		{
-			Questions = new HashSet<AssignmentQuestion>();
+			Questions = new HashSet<ExamQuestion>();
 		}
 	}
 }