The relay serves WS /camera/<id>/ws from the shared camera feed, and the offline picture when the Pi camera is down #139

Part of #87.

Done:

• [ ] Software/so101/src/relay.ts upgrades /camera/<id>/ws for a known camera id and refuses unknown ids. It reads src/camera-feed.ts once, however many people watch, through src/camera-socket.ts.
• [ ] When the upstream stalls or is unreachable, the socket carries the offline frame (src/offline-frame.ts).
• [ ] Tests with a fake MJPEG upstream: two viewers get whole JPEGs from one upstream connection; a stalled upstream gives the offline frame; an unknown id is refused.
• [ ] forge.json is unchanged; main's health / already deploys without the Pi. bun --cwd Software/so101 test and bun x tsc --noEmit pass.