Runs daily at 02:00 (and on workflow_dispatch). Builds the prod
compose stack with BuildKit, writes a transient .env.staging from
Gitea secrets, then `docker compose up -d --wait` so the job fails
loudly if any service's healthcheck never reports healthy.
The --profile staging flag starts the mailpit catcher in place of
a real SMTP relay; no production SMTP credentials touch the staging
environment.
The .env.staging file is cleaned up in `if: always()` to avoid
leaving secrets in the runner workspace between runs.
Refs #497.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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