Defines topic hierarchy, payload schemas, QoS levels, heartbeat
protocol, camera auto-discovery via announce topic, offline
buffering strategy, and command/response flow for start/stop.
Architecture: travel router subnet (192.168.4.x), Pi Zero 2 W
runs Mosquitto + Go backend, ESP32s dual-STA to GoPro AP +
travel router. No internet dependency.
Closes CUB-238.
