[Security] Add secure install and slow query log support

[alessekhjoy1824-automation]

This PR enhances MySQL security and introduces slow query log auditing capabilities for compliance, based on geerlingguy/ansible-role-mysql.

Security hardening items:

• Disable remote root logins
• Remove anonymous users
• Remove default test database

Slow query log configuration logic:

• Controlled via mysql_slow_query_log_enabled (default: false)
• Generates /etc/mysql/conf.d/slow-query.cnf when enabled
• slow_query_log is set to ON/OFF based on the variable
• long_query_time defaults to 2 seconds but can be overridden via mysql_slow_query_time
• Logs written to /var/log/mysql/slow.log with proper ownership

Compatibility:

• New features are disabled by default and guarded behind boolean variables
• No changes to handlers/, templates/, or vars/ directories

Validation steps:

• Branch feat/mysql-security-audit contains all changes
• Includes are added after configure and before other tasks
• YAML files are linted for basic formatting

[alessekhjoy1824-automation]

安全加固：该 PR 包含移除匿名用户、禁用远程 root 登录、删除测试数据库等措施，以提升 MySQL 默认安全性。

[alessekhjoy1824-automation]

慢查询日志：该 PR 新增 slow query log 配置（默认关闭），可通过变量开启并设置 long_query_time，日志输出至 /var/log/mysql/slow.log。