Stability checkpoint — GitHub Actions integration-tests live

- fix(gh-actions): run integration-tests inside container so services are reachable via alias (postgres:5432, kafka:9092). Without container the host-runner only saw localhost:port and KAFKA_CFG_ADVERTISED_LISTENERS broke container-to-container traffic.

CI :
- ✅ GitLab main pipeline #2514088467 green
- ✅ GitHub Actions run #25639568671 green (first successful integration-tests run on GH)
- ✅ Auto-merge dev → main (template iris-common) pushed dev to main without manual MR

Both runners now run integration-tests on every push :
- GitLab CI on saas-linux-medium-amd64 (~3 min, ~400 free min/mo)
- GitHub Actions on ubuntu-latest container ghcr.io/astral-sh/uv:python3.14 (~3 min, ~2000 free min/mo for public repos)

Double validation = belt-and-braces : GitLab is the reference (rest of CI runs there), GitHub Actions is the backup gate.

- GitLab CI integration-tests on SaaS quota (re-test local-only with redpanda is a candidate next session)
- Docker image alpine 412 MB still blocked

- Re-test integration-tests on macbook-local runner with redpanda image (no longer blocked by 30s kafka KRaft timeout)
- Promote redpanda usage to local dev testcontainers for faster pytest