1.3.0: Ops: healthcheck on the worker container

arq publishes a heartbeat in Redis on every cycle; ``python -m arq …
--check`` reads it and exits 0 iff fresh. Wired as a compose
healthcheck so the container flips to unhealthy if Redis becomes
unreachable or the worker stops pumping (silent crash, deadlock,
restart-loop catcher upstream).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>