Integrate pgopr into the composition where relevant

[fheikens]

Part of #1 (upstream split).

Once the pgagroal + pgexporter integration (#42) is stable, layer pgopr into the composition where it adds value. pgopr is a Kubernetes operator with images in pgopr/pgopr-images (work-in-progress upstream), so this is a later phase that follows the maturity of those images.

Scope (provisional — refine when picked up)

• Evaluate whether pgopr fits the Compose model or only the Helm/K8s path.
• If Compose-appropriate, add an example composition; otherwise document the K8s/Helm integration path.

Acceptance criteria

• Decision recorded on whether pgopr belongs in the Compose example or only the K8s path.
• Integration example or documented path added accordingly.
• Specification + tests per STDD before implementation.

Rationale

Completes the three-component split (#1). Sequenced last because pgopr/pgopr-images is still WIP upstream.