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

Entities/DTO/Submission/SubmissionDetailDto.cs

@@ -0,0 +1,21 @@
+using Entities.Contracts;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+	public class SubmissionDetailDto
+	{
+		public Guid Id { get; set; } = Guid.Empty;
+		public Guid SubmissionId { get; set; }
+		public Guid StudentId { get; set; }
+		public Guid ExamQuestionId { get; set; }
+		public string? StudentAnswer { get; set; }
+		public bool? IsCorrect { get; set; }
+		public float? PointsAwarded { get; set; }
+		public string? TeacherFeedback { get; set; }
+	}
+}