Add /health endpoint for Render service health checks

[Copilot]

Render requires an HTTP endpoint it can poll to determine service availability. The app did not expose a dedicated health-check route, so platform health monitoring could not be configured reliably.

• Server health-check route

  • Added a minimal Express endpoint at GET /health in server.js.
  • Returns HTTP 200 with a lightweight OK response body for periodic Render probes.

• Deployment documentation update

  • Updated the README deployment section to specify the Render health check path as /health.

app.get('/health', (req, res) => {
    res.status(200).send('OK');
});

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