Allow Docker to distinguish a running container from one actually
serving traffic by adding a /health endpoint that pings the database.
Returns 200 when healthy, 503 when the DB connection is lost.
Add HEALTHCHECK to Dockerfile and healthcheck config to the server
service in docker-compose.yml. Also add start_period to the existing
db healthcheck for consistency.
Multi-stage Dockerfile for smaller runtime image, CD workflow triggers
on main branch pushes and version tags, docker-compose defaults to the
prebuilt GHCR image.
