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

TechHelper.Server/Repository/TextbookRepository.cs

@@ -0,0 +1,21 @@
+using Entities.Contracts;
+using Microsoft.EntityFrameworkCore;
+using SharedDATA.Api;
+using TechHelper.Context;
+
+namespace TechHelper.Repository
+{
+    /// <summary>
+    /// 教材仓储实现类
+    /// </summary>
+    public class TextbookRepository : Repository<Textbook>, IRepository<Textbook>
+    {
+        /// <summary>
+        /// 初始化教材仓储
+        /// </summary>
+        /// <param name="dbContext">数据库上下文</param>
+        public TextbookRepository(ApplicationContext dbContext) : base(dbContext)
+        {
+        }
+    }
+}