refactor(observability): decouple Grafana from Manage

- Replace embedded Grafana iframes in ObservabilityPage with external
  Grafana link cards. Grafana URL is configurable via VITE_GRAFANA_URL
  (frontend) or GRAFANA_URL (backend env exposed to frontend build).
- Remove the grafana scrape job from monitoring/prometheus/prometheus.yml
  so the main Prometheus stack no longer depends on Grafana.
- Pass GRAFANA_URL through docker-compose.yml and docker-compose.dev.yml.

docker-compose.yml

@@ -27,6 +27,7 @@ services:
       SSH_KNOWN_HOSTS_PATH: /app/backend/.cache/known_hosts
       PROMETHEUS_FILE_SD_DIR: ${PROMETHEUS_FILE_SD_DIR:-/app/backend/.cache/prometheus-file-sd}
       ALERTMANAGER_URL: ${ALERTMANAGER_URL:-http://alertmanager:9093}
+      GRAFANA_URL: ${GRAFANA_URL:-http://grafana:3000}
     volumes:
       - ${BACKEND_CACHE_DIR:-./backend-cache}:/app/backend/.cache
     restart: unless-stopped