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

TechHelper.Server/Services/Textbook/ITextbookService.cs

@@ -0,0 +1,12 @@
+using Entities.DTO;
+using TechHelper.Services;
+
+namespace TechHelper.Services.Beta
+{
+    /// <summary>
+    /// 教材服务接口
+    /// </summary>
+    public interface ITextbookService : IBaseService<TextbookDto, Guid>
+    {
+    }
+}