Add scan cancellation feature

- 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

app/web/services/scan_service.py

@@ -257,6 +257,9 @@ class ScanService:
         elif scan.status == 'failed':
             status_info['progress'] = 'Failed'
             status_info['error_message'] = scan.error_message
+        elif scan.status == 'cancelled':
+            status_info['progress'] = 'Cancelled'
+            status_info['error_message'] = scan.error_message
 
         return status_info