No health check endpoint for the API service

[Haroldwonder]

The backend service has a health check endpoint (/health) but the API service (api/src/index.ts) does not expose a health check endpoint. Load balancers and container orchestrators cannot determine if the API service is healthy.

File: api/src/index.ts, api/src/app.ts

Fix: Add a GET /health endpoint to the API service that returns 200 OK with service status, database connectivity, and contract reachability.