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

Entities/DTO/Submission/StudentSubmissionSummaryDto.cs

@@ -0,0 +1,22 @@
+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; }
+    }
+}