Health & readiness probes

`GET /health` (app up) and `GET /ready` (DB reachable). Useful for containers.

Look for the Express app setup in the main server file (likely index.ts or app.ts). Add two new GET routes: '/health' that returns a simple 200 OK, and '/ready' that checks the database connection (e.g., a simple query). Test by running the server and curling the endpoints. Ensure the responses are appropriate for container health checks.

索引モデルが issue の本文から書いたものです。