CI: Pin Azure deploy to commit SHA, prune images, make Prometheus private

[DoPri]

What does this PR do?

Brings the Azure VM deploy up to the same rigor the Kubernetes path already has, and closes an open Prometheus on the public deployment.

Pin deployed images to the commit SHA + prune old ones. The k8s deploy passes image.tag=<sha> to Helm, but the VM just did docker compose pull against ghcr.io/...:latest, so it ran "whatever latest was at pull time": not traceable to a commit, racy when two merges queue, and impossible to roll back precisely.

• docker-compose.yml now uses :${IMAGE_TAG:-latest} for the five app images, so it still defaults to :latest for local/manual use.
• The Ansible deploy role passes IMAGE_TAG (from the new image_tag var) into docker compose pull and up, defaulting to latest when unset.
• The CI Deploy to Azure VM job passes the built short SHA (needs.build-image.outputs.image-tag, the same value k8s already uses) as image_tag.
• The role also prunes unused images older than a week (docker image prune -af --filter until=168h) after each deploy, so old SHA-tagged layers stop piling up until the disk fills.

Keep Prometheus off the public router. On the public (Let's Encrypt) deployment, /prometheus was reachable unauthenticated: the TLS overlay only added TLS, no auth. In k8s Prometheus is private (port-forward only) and only Grafana is exposed (behind Keycloak). The overlay now disables the Prometheus Traefik router and publishes it on the VM's loopback (127.0.0.1:9090), so it's reachable via an SSH tunnel (ssh -L 9090:localhost:9090 <vm>), mirroring kubectl port-forward. Local dev is unchanged: it doesn't use the overlay, so http://localhost/prometheus/ still works.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Docs
• CI / infra

Definition of Done

• CI is green
• If the API changed: api/openapi.yaml is updated and lint passes
• If a service was added or restructured: docker-compose still comes up cleanly (docker compose up)
• Tests added or updated for the changed behaviour
• Docs updated where it matters (README, ADRs, comments)

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📥 Commits

Reviewing files that changed from the base of the PR and between e76d670 and d6fe99a.

📒 Files selected for processing (6)

• .env.example
• .github/workflows/ci.yml
• docker-compose.letsencrypt.yml
• docker-compose.yml
• infra/azure/README.md
• infra/azure/ansible/roles/deploy/tasks/main.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/azure-deploy-rigor

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[ladu-tu]

Thanks for all the cleanup work!