Find the camera on the network instead of asking for its address

The add-camera form asked for an IP address, and a shop owner does not
know their camera's IP address - it is on a sticker under the camera or
in a menu that differs by make. That field is where onboarding stopped
for anyone who was not an installer.

behavision/discover.py: one ONVIF WS-Discovery multicast (names the
camera and often its make) merged with a TCP sweep of port 554 across
the local /24 (misses nothing that streams). Stdlib only, ~4 s on the
office network, both cameras found. The add-camera sheet leads with
'Find cameras on this network'; picking a row fills the address and,
when the make is recognisable, the stream path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KGcjxF1cNLcuwc3DAPcnfj

desktop/frontend/src/bridge.js

@@ -32,6 +32,7 @@ export const api = {
 
   cameras:            () => call('Cameras'),
   testCamera:         (cam) => call('TestCamera', cam),
+  discoverCameras:    () => call('DiscoverCameras'),
   saveCamera:         (id, cam) => call('SaveCamera', id, cam),
   deleteCamera:       (id) => call('DeleteCamera', id),
   startPlacement:     (id, seconds) => call('StartPlacementCheck', id, seconds),