feat: add RTSP/IP camera support via --camera URL

[johnoct]

Summary

• Add ThreadedVideoCapture class that reads frames in a background thread (latest-frame pattern, prevents RTSP lag)
• Add open_camera_source() factory function that picks local VideoCapture or ThreadedVideoCapture based on source string
• Change --camera from int-only to accept RTSP/HTTP URLs: --camera rtsp://user:pass@192.168.1.100:554/stream1
• Add --rtsp-transport tcp|udp flag (default: tcp)
• Mask credentials in all log output via mask_credentials()
• Update reconnect_camera() to handle both local and network sources

Backward compatible: --camera 0 still works exactly as before.

Test plan

• All 290 tests pass (31 new)
• ThreadedVideoCapture: init, read, copy semantics, release, is_healthy, timeout, RTSP env var
• open_camera_source: local index, RTSP URL, HTTP URL, invalid source, not-found exits
• mask_credentials: with/without credentials, HTTP, local index
• _is_network_source: RTSP, HTTP, HTTPS, local index, integer
• parse_args: camera accepts URL, rtsp_transport choices, defaults
• reconnect_camera: local and RTSP reconnection, failure returns None
• Integration tests updated for new function names