Stability checkpoint — GitHub Actions backup CI (double-CI per ADR-0069)

- feat(ci): backup build-test workflow via GitHub Actions (mvn verify + postgres service)

- ✅ Main pipeline #2518082724 success
- ✅ GH Actions run #25718233396 success (first live double-CI run)
- ✅ Auto-merge dev→main fires on .github/** changes (workflow rules allowlist includes the path)

- 🔄 CI/CD : Linux amd64 native backup gate via GH Actions (ubuntu-latest)
- ✅ Qualité : independent failure domain from GitLab SaaS