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

Entities/DTO/Submission/StudentSubmissionDetailDto.cs

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+using Entities.Contracts;
+using System;
+using System.Collections.Generic;
+
+namespace Entities.DTO
+{
+    public class StudentSubmissionDetailDto
+    {
+        // 基本信息
+        public Guid Id { get; set; }
+        public Guid AssignmentId { get; set; }
+        public Guid StudentId { get; set; }
+        public DateTime SubmissionTime { get; set; }
+        public float OverallGrade { get; set; }
+        public string OverallFeedback { get; set; } = string.Empty;
+        public SubmissionStatus Status { get; set; }
+        
+        // Assignment信息
+        public ExamDto Assignment { get; set; } = new ExamDto();
+        
+        // 错误分析
+        public Dictionary<string, int> ErrorTypeDistribution { get; set; } = new Dictionary<string, int>();
+        public Dictionary<string, float> ErrorTypeScoreDistribution { get; set; } = new Dictionary<string, float>();
+        
+        // 成绩统计
+        public int TotalRank { get; set; }
+        public List<float> AllScores { get; set; } = new List<float>();
+        public float AverageScore { get; set; }
+        public float ClassAverageScore { get; set; }
+        
+        // 课文分布
+        public Dictionary<string, int> LessonErrorDistribution { get; set; } = new Dictionary<string, int>();
+        public Dictionary<string, int> KeyPointErrorDistribution { get; set; } = new Dictionary<string, int>();
+        
+        // 基础统计
+        public int TotalQuestions { get; set; }
+        public int CorrectCount { get; set; }
+        public int ErrorCount { get; set; }
+        public float AccuracyRate { get; set; }
+    }
+}

Entities/DTO/Submission/StudentSubmissionSummaryDto.cs

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+using System;
+
+namespace Entities.DTO
+{
+    public class StudentSubmissionSummaryDto
+    {
+        public Guid Id { get; set; }
+        public string AssignmentName { get; set; }
+        public int ErrorCount { get; set; }
+        public DateTime CreatedDate { get; set; }
+        public float Score { get; set; }
+        public int TotalQuestions { get; set; }
+        public string StudentName { get; set; }
+        public string Status { get; set; }
+    }
+
+    public class StudentSubmissionSummaryResponseDto
+    {
+        public List<StudentSubmissionSummaryDto> Submissions { get; set; }
+        public int TotalCount { get; set; }
+    }
+}

Entities/DTO/Submission/SubmissionDetailDto.cs

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+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; }
+	}
+}

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>();
+	}
+}

Entities/DTO/Submission/SubmissionListDto.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+	public record SubmissionListDto
+	{
+		public Guid Id { get; init; }
+
+
+		public float OverallGrade { get; init; }
+		public int ErrorCount { get; init; }
+		public Dictionary<string, int>? ErrorQuestionTypeDistribution { get; init; }
+		public Dictionary<string, int>? ErrorQuestionLessonDistribution { get; init; }
+
+
+		public byte AttemptNumber { get; init; }
+	}
+}

Entities/DTO/Submission/SubmissionUpdateDto.cs

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+using Entities.Contracts;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+	public record SubmissionDetailTeacherUpdateDto(Guid Id, bool IsCorrect, float? PointsAwarded);
+	public record SubmissionTeacherUpdateDto(Guid Id, float OverallGrade, string? OverallFeedback, SubmissionStatus Status, ICollection<SubmissionDetailTeacherUpdateDto> SubmissionUpdateDetails);
+}