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/utils/validators.py

@@ -23,7 +23,7 @@ def validate_scan_status(status: str) -> tuple[bool, Optional[str]]:
         >>> validate_scan_status('invalid')
         (False, 'Invalid status: invalid. Must be one of: running, completed, failed')
     """
-    valid_statuses = ['running', 'completed', 'failed']
+    valid_statuses = ['running', 'completed', 'failed', 'cancelled']
 
     if status not in valid_statuses:
         return False, f'Invalid status: {status}. Must be one of: {", ".join(valid_statuses)}'