feat: upgrade node to e2-standard-4 (4 vCPU / 16 GB)

Increase from e2-medium (2 vCPU / 4 GB) to e2-standard-4 to give
the full stack (app + kube-prometheus-stack + nginx ingress) enough
CPU headroom and avoid API rate limiter exhaustion on startup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>