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/server.js

@@ -1,7 +1,6 @@
 const express = require('express');
 const cors = require('cors');
 const session = require('express-session');
-// const MySQLStore = require('express-mysql-session')(session);
 const path = require('path');
 require('dotenv').config();
 
@@ -27,26 +26,9 @@ app.use(cors({
 app.use(express.json());
 app.use(express.urlencoded({ extended: true }));
 
-// Session
-/*
-const sessionStore = new MySQLStore({
-    expiration: 86400000,
-    createDatabaseTable: true,
-    schema: {
-        tableName: 'sessions',
-        columnNames: {
-            session_id: 'session_id',
-            expires: 'expires',
-            data: 'data'
-        }
-    }
-}, db.pool);
-*/
-
 app.use(session({
     key: 'session_cookie',
     secret: process.env.SESSION_SECRET || 'your-secret-key',
-    // store: sessionStore, // Use MemoryStore for SQLite migration simplification
     resave: false,
     saveUninitialized: false,
     cookie: {