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

Entities/DTO/Submission/SubmissionDto.cs

@@ -0,0 +1,23 @@
+using Entities.Contracts;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+	public class SubmissionDto
+	{
+		public Guid Id { get; set; } = Guid.Empty;
+		public Guid ExamId { get; set; }
+		public Guid StudentId { get; set; }
+		public DateTime SubmissionTime { get; set; }
+		public float OverallGrade { get; set; } = 0;
+		public string OverallFeedback { get; set; } = string.Empty;
+		public Guid? GraderId { get; set; }
+		public DateTime? GradedAt { get; set; }
+		public SubmissionStatus Status { get; set; }
+		public List<SubmissionDetailDto> SubmissionDetails { get; set; } = new List<SubmissionDetailDto>();
+	}
+}