Webcam support

• Webcam dependencies made optional — aiohttp and opencv-python-headlessmoved to[project.optional-dependencies] (pip install arcsecond[webcam]`)
• Proxy robustness — asyncio.get_running_loop(), clean disconnect handling, device-loss error messages, /health endpoint
• Docker compose — removed hardcoded WEBCAM_PROXY_URL (default lives in backend code), kept extra_hosts for Linux compatibility
• Setup command — always writes the latest compose file instead of asking users to diff/merge
• Documentation — new Webcam Proxy page with install, usage, and remote setup instructions