API: Prometheus metrics, DB pool tuning, header/contract docs, health/ready split - #374

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11, actions/setup-node@60edb5dd545a775178f52524783378180af0d1f8. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/