discover a camera when none is configured

CAPTURE_PATH defaulted to index 0, which is a coin flip on a machine with one
camera. A UVC camera exposes a second device node for its metadata stream --
/dev/video1 beside /dev/video0 -- and that node opens happily and never yields
an image, so the lowest device number is wrong about as often as it is right.

With neither CAMERA_ID nor CAPTURE_PATH set, the node now opens each
/dev/video* in numeric order and keeps the first that actually delivers a
frame. Reading one is the test, because opening is not evidence.

Discovery finding nothing falls back to index 0 rather than raising, which
keeps the failure in one place: the first read, where CI publishes a
placeholder and a normal run raises with a message naming the device.

CAMERA_ID is still worth preferring on a robot. Neither an index nor discovery
order is stable across reboots or across which port a camera is in; a by-id
name is.

dora_webcam/main.py

@@ -24,7 +24,7 @@ def main() -> None:
     """Open the camera and publish one frame per tick."""
     args = _parse_args()
 
-    device = camera.resolve(os.getenv("CAMERA_ID"), os.getenv("CAPTURE_PATH", "0"))
+    device = camera.resolve(os.getenv("CAMERA_ID"), os.getenv("CAPTURE_PATH"))
     capture = cv2.VideoCapture(device)
 
     width = _int("IMAGE_WIDTH")