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

TechHelper.Server/Context/DbInitializer.cs

@@ -0,0 +1,22 @@
+using Entities.Contracts;
+using Microsoft.AspNetCore.Identity;
+
+namespace TechHelper.Server.Context
+{
+	public class DbInitializer
+	{
+		public static async Task SeedRoles(RoleManager<IdentityRole<Guid>> roleManager)
+		{
+			string[] defaultRoleName = { "Admin", "Teacher", "Student" };
+
+			foreach (var roleName in defaultRoleName)
+			{
+				if (!await roleManager.RoleExistsAsync(roleName))
+				{
+					await roleManager.CreateAsync(new IdentityRole<Guid>(roleName));
+					Console.WriteLine($"Default role '{roleName}' created successfully.");
+				}
+			}
+		}
+	}
+}