Both frontend and backend now write rotating log files to
/mnt/converter/logs/ (shared between host and LXC):
/mnt/converter/logs/vm-bench.log (frontend)
/mnt/converter/logs/vm-bench-backend.log (backend)
- RotatingFileHandler: 10 MB per file, 5 backups
- Console handler still writes to systemd journal
- Logs/ directory is gitignored and auto-created on startup
- Install script creates logs/ directory
