Distilled from the house pattern across sibling projects: tag push +
workflow_dispatch triggers, REGISTRY_TOKEN login, raw docker build/push
to gitea.home.hrajfrisbee.cz. Adds a lightweight test gate, an immutable
sha-<12> tag, :latest only on real tag pushes, and a concurrency group.
Co-Authored-By: Claude <noreply@anthropic.com>
