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/mock.js

@@ -47,6 +47,11 @@ if (scenario) {
     StartEngine: () => delay({state: 'running', reachable: true}),
     StopEngine: () => delay({state: 'stopped', reachable: false}),
     Cameras: () => delay(st.cameras.map(c => ({...c, connected: true, frames: 1200, faces: 9}))),
+    DiscoverCameras: () => delay({networks: ['192.168.1.0/24'], cameras: [
+      {host: '192.168.1.122', rtsp: true, onvif: true, name: 'HIKVISION DS-2CD2043G2', make: 'hikvision'},
+      {host: '192.168.1.121', rtsp: true, onvif: true, name: 'IPC-model IPC', make: ''},
+      {host: '192.168.1.40', rtsp: true, onvif: false, name: '', make: ''},
+    ]}, 2500),
     TestCamera: (cam) => delay({ok: Boolean(cam.host), width: 800, height: 448, codec: 'hevc', error: cam.host ? '' : 'no host'}, 1500),
     SaveCamera: (id, cam) => { const c = {id: id || cam.id || 'cam' + (st.cameras.length + 1), ...cam, has_password: Boolean(cam.password)}; delete c.password; st.cameras = [...st.cameras.filter(x => x.id !== c.id), c]; return delay(c) },
     DeleteCamera: (id) => { st.cameras = st.cameras.filter(c => c.id !== id); return delay(null) },