refactor(backend): 更新数据库配置并迁移至MySQL，优化文档和技术栈描述

backend/.env.example

@@ -4,8 +4,12 @@ PORT=3000
 HOST=0.0.0.0
 
 # 数据库配置
-MONGODB_URI=mongodb://localhost:27017/jiebanke
-MONGODB_URI_TEST=mongodb://localhost:27017/jiebanke_test
+DB_HOST=localhost
+DB_PORT=3306
+DB_USER=root
+DB_PASSWORD=your-mysql-password
+DB_NAME=jiebandata
+DB_NAME_TEST=jiebandata_test
 
 # JWT配置
 JWT_SECRET=your-super-secret-jwt-key-change-this-in-production
@@ -30,6 +34,11 @@ REDIS_HOST=localhost
 REDIS_PORT=6379
 REDIS_PASSWORD=
 
+# MySQL连接池配置
+DB_CONNECTION_LIMIT=10
+DB_CHARSET=utf8mb4
+DB_TIMEZONE=+08:00
+
 # 第三方API配置
 MAP_API_KEY=your-map-api-key
 SMS_API_KEY=your-sms-api-key