feat: 实现教师资料更新、操作日志和系统设置功能

新增教师资料更新功能，包括个人信息修改和密码更新
添加操作日志记录系统，记录用户关键操作
实现系统设置模块，支持动态配置系统参数
重构数据库模型，新增教师表和系统设置表
优化成绩录入逻辑，支持平时分、期中和期末成绩计算
添加数据导出功能，支持学生、教师和成绩数据导出
完善管理员后台，增加统计图表和操作日志查看

backend/models/Teacher.js

@@ -0,0 +1,38 @@
+const db = require('../config/database');
+
+class Teacher {
+    static async findById(id) {
+        const teachers = await db.query('SELECT * FROM teachers WHERE id = ?', [id]);
+        return teachers[0];
+    }
+
+    static async create(teacherData) {
+        const { id, name, department, title, contact_info } = teacherData;
+        await db.query(
+            'INSERT INTO teachers (id, name, department, title, contact_info) VALUES (?, ?, ?, ?, ?)',
+            [id, name, department, title, contact_info]
+        );
+    }
+
+    static async update(id, data) {
+        const fields = [];
+        const values = [];
+        if (data.name) { fields.push('name = ?'); values.push(data.name); }
+        if (data.department) { fields.push('department = ?'); values.push(data.department); }
+        if (data.title !== undefined) { fields.push('title = ?'); values.push(data.title); }
+        if (data.contact_info !== undefined) { fields.push('contact_info = ?'); values.push(data.contact_info); }
+
+        if (fields.length === 0) return true;
+
+        values.push(id);
+        const sql = `UPDATE teachers SET ${fields.join(', ')} WHERE id = ?`;
+        await db.query(sql, values);
+        return true;
+    }
+
+    static async delete(id) {
+        await db.query('DELETE FROM teachers WHERE id = ?', [id]);
+    }
+}
+
+module.exports = Teacher;