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

Entities/DTO/Class/ClassValidDto.cs

@@ -0,0 +1,10 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Entities.DTO
+{
+	public record ClassValidDto(string SchoolName, byte Grade, byte Class);
+}