feat: non-blocking scan with progressive device discovery

Backend:
- asyncio.to_thread(_nmap_scan) — nmap no longer blocks the event loop
- Commit each discovered device immediately (previously one bulk commit at end)
- Update ScanRun.devices_found after each device so history panel shows live count
- broadcast_scan_update() pushes {type: scan_device_found} WS event per device
- Refactor broadcast_status to shared _broadcast() helper, adds type: "status" field

Frontend:
- useStatusPolling handles both WS message types (status / scan_device_found)
- canvasStore: scanEventTs + notifyScanDeviceFound() action
- PendingDevicesPanel auto-refreshes when WS scan event is received

frontend/src/hooks/useStatusPolling.ts

@@ -3,15 +3,18 @@ import { useCanvasStore } from '@/stores/canvasStore'
 import { useAuthStore } from '@/stores/authStore'
 
 interface StatusMessage {
-  node_id: string
-  status: 'online' | 'offline' | 'pending' | 'unknown'
-  checked_at: string
-  response_time_ms: number | null
+  type?: string
+  node_id?: string
+  status?: 'online' | 'offline' | 'pending' | 'unknown'
+  checked_at?: string
+  response_time_ms?: number | null
+  run_id?: string
+  devices_found?: number
 }
 
 export function useStatusPolling() {
   const wsRef = useRef<WebSocket | null>(null)
-  const { updateNode } = useCanvasStore()
+  const { updateNode, notifyScanDeviceFound } = useCanvasStore()
   const { isAuthenticated, token } = useAuthStore()
 
   useEffect(() => {
@@ -27,11 +30,15 @@ export function useStatusPolling() {
     ws.onmessage = (event) => {
       try {
         const msg: StatusMessage = JSON.parse(event.data)
-        updateNode(msg.node_id, {
-          status: msg.status,
-          response_time_ms: msg.response_time_ms ?? undefined,
-          last_seen: msg.status === 'online' ? msg.checked_at : undefined,
-        })
+        if (msg.type === 'scan_device_found') {
+          notifyScanDeviceFound()
+        } else if (msg.node_id && msg.status) {
+          updateNode(msg.node_id, {
+            status: msg.status,
+            response_time_ms: msg.response_time_ms ?? undefined,
+            last_seen: msg.status === 'online' ? msg.checked_at : undefined,
+          })
+        }
       } catch {
         // ignore malformed messages
       }
@@ -45,5 +52,5 @@ export function useStatusPolling() {
       ws.close()
       wsRef.current = null
     }
-  }, [isAuthenticated, token, updateNode])
+  }, [isAuthenticated, token, updateNode, notifyScanDeviceFound])
 }