docs: update DOCS.md with frontend, backend tests and monitoring stack

- Frontend: document API_BASE relative URL approach and Vite dev proxy
- Backend: add section 6 covering Jest/supertest test suite (9 tests)
- Repository structure: add metrics.js, __tests__/, k8s/monitoring/,
  infra/iam.tf to the tree
- Pipeline: add deploy-monitoring job, GRAFANA_ADMIN_PASSWORD variable,
  update SA role to roles/container.admin
- Observability: replace basic GCP logging section with full section 9
  covering Prometheus/Grafana stack, instrumented metrics, dashboard
  panels with PromQL, Grafana access via ExternalName service

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