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

behavision/api.py

@@ -332,6 +332,17 @@ def create_app(engine: Engine) -> FastAPI:
             worker.commission.cancel()
         return {"cancelled": camera_id}
 
+    @app.get("/api/cameras/discover")
+    def discover_cameras() -> dict:
+        """Cameras on this PC's network, for the add-camera form to pick from.
+
+        A sync def so FastAPI runs it in the threadpool: it holds a socket
+        open for a couple of seconds and sweeps a /24, and the event loop
+        must keep serving the live picture meanwhile.
+        """
+        from .discover import discover
+        return discover()
+
     @app.post("/api/cameras/test")
     def test_camera(payload: CameraPayload) -> dict:
         """Try a camera WITHOUT saving it - the UI's Test button.