Add Render deployment configuration and documentation

[franciscojavierarceo]

Parent: #102

Provide a Render deployment path for the gateway.

Scope:

• Render Blueprint or equivalent service configuration.
• Managed PostgreSQL wiring and environment-variable/secret setup.
• External inference endpoint configuration.
• Health check, WebSocket/streaming behavior, and startup/migration guidance.
• Document limitations and production considerations compared with Kubernetes.

Acceptance: a fresh Render deployment can start the gateway, connect to PostgreSQL and inference, and preserve conversations across redeploys.

See the parent issue: #102