fix: use process health for Render liveness

[EcstaceeLOR]

What changed

• point Render's platform health check at the dependency-free process health endpoint
• retain /v1/ready for Postgres and Redis readiness diagnostics
• add a regression test that prevents the Blueprint from coupling liveness to free-tier datastores

Why

Render logs showed healthy gateway processes being replaced after datastore-backed readiness probes stalled. The replacements caused sustained public 502 responses.

Validation

• liveness Blueprint regression passed
• deployment-policy tests passed
• lint, formatting, repository policy, and OpenAPI coverage passed