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/internal/local/client.go

@@ -105,6 +105,12 @@ func (c *Client) DeleteCamera(ctx context.Context, id string) error {
 	return c.do(ctx, http.MethodDelete, "/api/cameras/"+id, nil, nil)
 }
 
+// DiscoverCameras asks the engine to scan the shop's network. A few seconds.
+func (c *Client) DiscoverCameras(ctx context.Context) (map[string]any, error) {
+	var out map[string]any
+	return out, c.do(ctx, http.MethodGet, "/api/cameras/discover", nil, &out)
+}
+
 func (c *Client) TestCamera(ctx context.Context, cam map[string]any) (map[string]any, error) {
 	var out map[string]any
 	return out, c.do(ctx, http.MethodPost, "/api/cameras/test", cam, &out)