feat: 添加学生提交系统功能

- 添加学生提交管理服务 (StudentSubmissionService, StudentSubmissionDetailService)
- 新增学生提交相关控制器 (StudentSubmissionController, StudentSubmissionDetailController)
- 添加学生提交数据传输对象 (StudentSubmissionDetailDto, StudentSubmissionSummaryDto)
- 新增学生提交相关页面组件 (StudentExamView, ExamDetailView, StudentCard等)
- 添加学生提交信息卡片组件 (SubmissionInfoCard, TeacherSubmissionInfoCard)
- 更新数据库迁移文件以支持提交系统

Entities/Contracts/Submission.cs

@@ -34,7 +34,7 @@ namespace Entities.Contracts
 		public DateTime SubmissionTime { get; set; }
 
 		[Column("overall_grade")]
-		public float? OverallGrade { get; set; }
+		public float OverallGrade { get; set; } = 0;
 
 		[Column("overall_feedback")]
 		public string? OverallFeedback { get; set; }