Switch the systemd ExecStartPost health check from docker exec + curl
to polling docker inspect for container health status. This piggybacks
on the container image's built-in HEALTHCHECK instead of duplicating it.

Also add a configurable container health interval (5s for Traefik setups,
15s otherwise) to speed up startup readiness detection without affecting
non-Traefik deployments.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>