Backend now publishes its OWN registry image (emi/thermograph-backend/app) via
build-push.yml and deploys ONLY the backend service to beta on push to main,
passing SERVICE=backend + BACKEND_IMAGE_TAG to infra's deploy.sh. Independent
of the frontend's pipeline -- the two ship asynchronously.
Claude-Session: https://claude.ai/code/session_01RdARHDJaYC1wSQRTYM6t3Z
