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

TechHelper.Server/Services/ClassService.cs

@@ -1,6 +1,7 @@
 using AutoMapper;
 using Entities.Contracts;
 using Entities.DTO;
+using Entities.DTO.Class;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.EntityFrameworkCore;
 using Org.BouncyCastle.Crypto;
@@ -41,7 +42,7 @@ namespace TechHelper.Services
 		}
 
 		// 实现 IBaseService<ClassDto, int>.DeleteAsync
-		public async Task<ApiResponse> DeleteAsync(Guid id) // ID 类型现在是 int
+		public async Task<ApiResponse> DeleteAsync(Guid id) 
 		{
 			try
 			{