feat: upgrade to e2-medium and right-size all resource limits

Full stack (app + monitoring + ingress + kube-system) requires ~860m
CPU and ~1.5GB RAM in requests. e2-small (2GB) had insufficient
headroom; e2-medium (2vCPU/4GB) provides ~45% CPU and ~55% RAM
utilization at request level, comfortable for production.

infra: e2-small → e2-medium
backend:    100m/500m CPU, 128Mi/256Mi RAM
frontend:   50m/200m CPU,  64Mi/128Mi RAM
prometheus: 100m/500m CPU, 400Mi/800Mi RAM
grafana:    50m/200m CPU,  128Mi/256Mi RAM

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