4.1-nightly-2025-12-27

feat(update): enhance backup filename security with random hash

- Add 8-char hex hash to backup filenames for unpredictability
- Implement caching of generated filename in Update class
- Update tests to validate new filename format with regex

BREAKING CHANGE: Backup filename format changed from
phpmyfaq-config-backup.YYYY-MM-DD.zip to
phpmyfaq-config-backup.YYYY-MM-DD.XXXXXXXX.zip