Add appointment reminders and audit log features

- Implemented a fallback trigger for appointment reminders in `reminders_tick.php`, allowing reminders to be sent in the background.
- Created an audit log page in `index.php` for administrators to view user activities, with filtering options for date, user, action, module, and status.
- Added functionality to log client-side actions in `log.php`, specifically for report exports.
- Developed a comprehensive students report in `students.php`, including filters for registration date, gender, and level, along with summary statistics and a detailed student list.
