refactor(database): 迁移到SQLite并清理旧MySQL相关代码

移除MySQL相关依赖和配置，完全迁移到SQLite数据库
删除不再需要的MySQL迁移脚本和备份文件
更新README文档说明新的数据库架构
在init_db.js中整合教师和系统设置表的初始化逻辑

backend/init_db.js

@@ -28,6 +28,13 @@ const insertStudent = async (student) => {
     );
 };
 
+const insertTeacher = async (teacher) => {
+    await run(
+        'INSERT INTO teachers (id, name, department, title, contact_info) VALUES (?, ?, ?, ?, ?)',
+        [teacher.id, teacher.name, teacher.department, teacher.title, teacher.contact_info]
+    );
+};
+
 const insertClass = async (cls) => {
     return await run(
         'INSERT INTO classes (class_name, grade, major, teacher_id) VALUES (?, ?, ?, ?)',
@@ -91,6 +98,8 @@ const init = async () => {
         await run('DROP TABLE IF EXISTS students');
         await run('DROP TABLE IF EXISTS users');
         await run('DROP TABLE IF EXISTS operation_logs');
+        await run('DROP TABLE IF EXISTS system_settings');
+        await run('DROP TABLE IF EXISTS teachers');
 
         // Create tables
         console.log('创建表结构...');
@@ -174,8 +183,38 @@ const init = async () => {
             )
         `);
 
+        await run(`
+            CREATE TABLE teachers (
+                id TEXT PRIMARY KEY,
+                name TEXT NOT NULL,
+                department TEXT,
+                title TEXT,
+                contact_info TEXT,
+                FOREIGN KEY (id) REFERENCES users(id) ON DELETE CASCADE
+            )
+        `);
+
+        await run(`
+            CREATE TABLE system_settings (
+                key TEXT PRIMARY KEY,
+                value TEXT
+            )
+        `);
+
         console.log('生成基础数据...');
 
+        // 0. System Settings
+        const defaultSettings = [
+            ['system_name', '学校成绩管理系统'],
+            ['current_semester', '2023-2024-2'],
+            ['allow_course_selection', '1'],
+            ['allow_grade_check', '1']
+        ];
+        
+        for (const [key, value] of defaultSettings) {
+            await run('INSERT OR IGNORE INTO system_settings (key, value) VALUES (?, ?)', [key, value]);
+        }
+
         // 1. Admin
         await insertUser({
             id: 'admin',
@@ -187,9 +226,16 @@ const init = async () => {
 
         // 2. Teachers (20 teachers)
         const teachers = [];
+        const departments = ['计算机学院', '软件学院', '信息工程学院', '理学院', '外国语学院'];
+        const titles = ['教授', '副教授', '讲师', '助教'];
+
         for (let i = 1; i <= 20; i++) {
             const id = `T${1000 + i}`;
             const name = `教师${String.fromCharCode(65 + (i % 26))}${i}`;
+            const dept = departments[i % departments.length];
+            const title = titles[i % titles.length];
+            const contact = `139${Math.floor(Math.random() * 90000000 + 10000000)}`;
+
             await insertUser({
                 id,
                 name,
@@ -197,6 +243,15 @@ const init = async () => {
                 role: 'teacher',
                 class: null
             });
+
+            await insertTeacher({
+                id,
+                name,
+                department: dept,
+                title: title,
+                contact_info: contact
+            });
+
             teachers.push(id);
         }