feat: 实现成绩管理系统核心功能

添加响应工具、错误处理中间件和数据库模型
创建用户、学生、课程和成绩相关服务
实现管理员、教师和学生控制器的基本功能
重构路由处理并优化数据库查询

backend/models/User.js

@@ -0,0 +1,33 @@
+const db = require('../config/database');
+const bcrypt = require('bcryptjs');
+
+class User {
+    static async findById(id) {
+        const users = await db.query('SELECT * FROM users WHERE id = ?', [id]);
+        return users[0];
+    }
+
+    static async findByIdAndRole(id, role) {
+        const users = await db.query('SELECT * FROM users WHERE id = ? AND role = ?', [id, role]);
+        return users[0];
+    }
+
+    static async create(userData) {
+        const { id, name, password, role, className } = userData;
+        const salt = await bcrypt.genSalt(10);
+        const hashedPassword = await bcrypt.hash(password, salt);
+        
+        await db.query(
+            'INSERT INTO users (id, name, password, role, class) VALUES (?, ?, ?, ?, ?)',
+            [id, name, hashedPassword, role, className || null]
+        );
+        
+        return { id, name, role, class: className };
+    }
+
+    static async verifyPassword(plainPassword, hashedPassword) {
+        return await bcrypt.compare(plainPassword, hashedPassword);
+    }
+}
+
+module.exports = User;