Introduce a mechanism to automatically pull the latest changes from the git repository when the homepage is accessed. This ensures the application stays up-to-date without manual intervention. The feature includes a lock file (`/tmp/hustoj_git_pull.lock`) to prevent excessive pulls, limiting execution to at most once every 60 seconds. The git pull command runs in the background to avoid blocking the page load, with output logged to `/tmp/hustoj_git_pull.log`.
- Added hardness filter dropdown in problemset UI with options 1-8
- Implemented server-side hardness parameter validation and filtering
- Refactored query parameter handling to use $filter_params array
- Updated SQL queries to use parameter binding for both search and hardness filters
- Fixed pagination to include hardness parameter in URL
- Improved code organization by centralizing filter parameter management
