Feat/paykit payment lock demo

[dzdidi]

Summary

Contract and risk impact

• Public API or SDK contract
• Persisted data or migration
• Authentication, authorization, identity, or secret handling
• Payment or entitlement behavior
• Runtime, deployment, or observability
• No contract/risk impact

Verification

docker compose --file compose.paykit-local-demo.yaml up -d --build

Documentation

Checklist

• The change is focused and self-reviewed.
• Regression tests were added where practical.
• No credentials, identities, private content, payment material, or generated local state are included.
• Formatting and relevant tests/lints pass.