- Replace subprocess.run() with Popen for cancellable processes
- Add cancel() method to SneakyScanner with process termination
- Track running scanners in registry for stop signal delivery
- Handle ScanCancelledError to set scan status to 'cancelled'
- Add POST /api/scans/<id>/stop endpoint
- Add 'cancelled' as valid scan status
- Add Stop button to scans list and detail views
- Show cancelled status with warning badge in UI
