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/Student.js

@@ -0,0 +1,23 @@
+const db = require('../config/database');
+
+class Student {
+    static async findByUserId(userId) {
+        const students = await db.query('SELECT * FROM students WHERE user_id = ?', [userId]);
+        return students[0];
+    }
+    
+    static async findById(studentId) {
+        const students = await db.query('SELECT * FROM students WHERE student_id = ?', [studentId]);
+        return students[0];
+    }
+
+    static async create(studentData) {
+        const { id, name, className, userId } = studentData;
+        await db.query(
+            'INSERT INTO students (id, name, class, user_id) VALUES (?, ?, ?, ?)',
+            [id, name, className, userId]
+        );
+    }
+}
+
+module.exports = Student;