Adds a GET `/api/relay/policy` health check endpoint and enforces the `RELAY_POLICY_SECRET` environment variable in production to secure the POST `/api/relay/policy` endpoint. Replit-Commit-Author: Agent Replit-Commit-Session-Id: 418bf6f8-212b-4bb0-a7a5-8231a061da4e Replit-Commit-Checkpoint-Type: full_checkpoint Replit-Commit-Event-Id: 7ee87f59-1dfd-4a71-8c6f-5938330c7b4a Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/9f85e954-647c-46a5-90a7-396e495a805a/418bf6f8-212b-4bb0-a7a5-8231a061da4e/Q83Uqvu Replit-Helium-Checkpoint-Created: true