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/scans.html

@@ -26,6 +26,7 @@
                             <option value="running">Running</option>
                             <option value="completed">Completed</option>
                             <option value="failed">Failed</option>
+                            <option value="cancelled">Cancelled</option>
                         </select>
                     </div>
                     <div class="col-md-4">
@@ -248,20 +249,27 @@
                 statusBadge = '<span class="badge badge-info">Running</span>';
             } 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>`;
             }
 
+            // Action buttons
+            let actionButtons = `<a href="/scans/${scan.id}" class="btn btn-sm btn-secondary">View</a>`;
+            if (scan.status === 'running') {
+                actionButtons += `<button class="btn btn-sm btn-warning ms-1" onclick="stopScan(${scan.id})">Stop</button>`;
+            } else {
+                actionButtons += `<button class="btn btn-sm btn-danger ms-1" onclick="deleteScan(${scan.id})">Delete</button>`;
+            }
+
             row.innerHTML = `
                 <td class="mono">${scan.id}</td>
                 <td>${scan.title || 'Untitled Scan'}</td>
                 <td class="text-muted">${timestamp}</td>
                 <td class="mono">${duration}</td>
                 <td>${statusBadge}</td>
-                <td>
-                    <a href="/scans/${scan.id}" class="btn btn-sm btn-secondary">View</a>
-                    ${scan.status !== 'running' ? `<button class="btn btn-sm btn-danger ms-1" onclick="deleteScan(${scan.id})">Delete</button>` : ''}
-                </td>
+                <td>${actionButtons}</td>
             `;
 
             tbody.appendChild(row);
@@ -489,6 +497,33 @@
         }
     }
 
+    // Stop scan
+    async function stopScan(scanId) {
+        if (!confirm(`Are you sure you want to stop scan ${scanId}?`)) {
+            return;
+        }
+
+        try {
+            const response = await fetch(`/api/scans/${scanId}/stop`, {
+                method: 'POST'
+            });
+
+            if (!response.ok) {
+                const data = await response.json();
+                throw new Error(data.message || 'Failed to stop scan');
+            }
+
+            // Show success message
+            showAlert('success', `Stop signal sent to scan ${scanId}.`);
+
+            // Refresh scans after a short delay
+            setTimeout(() => loadScans(), 1000);
+        } catch (error) {
+            console.error('Error stopping scan:', error);
+            showAlert('danger', `Failed to stop scan: ${error.message}`);
+        }
+    }
+
     // Delete scan
     async function deleteScan(scanId) {
         if (!confirm(`Are you sure you want to delete scan ${scanId}?`)) {