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/templates/scan_detail.html

@@ -20,6 +20,10 @@
                     <span id="refresh-text">Refresh</span>
                     <span id="refresh-spinner" class="spinner-border spinner-border-sm ms-1" style="display: none;"></span>
                 </button>
+                <button class="btn btn-warning ms-2" onclick="stopScan()" id="stop-btn" style="display: none;">
+                    <span id="stop-text">Stop Scan</span>
+                    <span id="stop-spinner" class="spinner-border spinner-border-sm ms-1" style="display: none;"></span>
+                </button>
                 <button class="btn btn-danger ms-2" onclick="deleteScan()" id="delete-btn">Delete Scan</button>
             </div>
         </div>
@@ -471,8 +475,11 @@
         } else if (scan.status === 'running') {
             statusBadge = '<span class="badge badge-info">Running</span>';
             document.getElementById('delete-btn').disabled = true;
+            document.getElementById('stop-btn').style.display = 'inline-block';
         } else if (scan.status === 'failed') {
             statusBadge = '<span class="badge badge-danger">Failed</span>';
+        } else if (scan.status === 'cancelled') {
+            statusBadge = '<span class="badge badge-warning">Cancelled</span>';
         } else {
             statusBadge = `<span class="badge badge-info">${scan.status}</span>`;
         }
@@ -697,6 +704,59 @@
         }
     }
 
+    // Stop scan
+    async function stopScan() {
+        if (!confirm(`Are you sure you want to stop scan ${scanId}?`)) {
+            return;
+        }
+
+        const stopBtn = document.getElementById('stop-btn');
+        const stopText = document.getElementById('stop-text');
+        const stopSpinner = document.getElementById('stop-spinner');
+
+        // Show loading state
+        stopBtn.disabled = true;
+        stopText.style.display = 'none';
+        stopSpinner.style.display = 'inline-block';
+
+        try {
+            const response = await fetch(`/api/scans/${scanId}/stop`, {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json'
+                }
+            });
+
+            if (!response.ok) {
+                let errorMessage = `HTTP ${response.status}: Failed to stop scan`;
+                try {
+                    const data = await response.json();
+                    errorMessage = data.message || errorMessage;
+                } catch (e) {
+                    // Ignore JSON parse errors
+                }
+                throw new Error(errorMessage);
+            }
+
+            // Show success message
+            showAlert('success', `Stop signal sent to scan ${scanId}.`);
+
+            // Refresh scan data after a short delay
+            setTimeout(() => {
+                loadScan();
+            }, 1000);
+
+        } catch (error) {
+            console.error('Error stopping scan:', error);
+            showAlert('danger', `Failed to stop scan: ${error.message}`);
+
+            // Re-enable button on error
+            stopBtn.disabled = false;
+            stopText.style.display = 'inline';
+            stopSpinner.style.display = 'none';
+        }
+    }
+
     // Find previous scan and show compare button
     let previousScanId = null;
     let currentConfigId = null;