Add coverage for health-server guards, db readiness deadlines, and lag histogram buckets - #1445

🛑 missing gitleaks license. Go grab one at gitleaks.io and store it as a GitHub Secret named GITLEAKS_LICENSE. For more info about the recent breaking update, see [here](https://github.com/gitleaks/gitleaks-action#-announcement).

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, gitleaks/gitleaks-action@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/