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.

monitoring/prometheus/prometheus.yml

@@ -41,11 +41,6 @@ scrape_configs:
       - targets:
           - alertmanager:9093
 
-  - job_name: grafana
-    static_configs:
-      - targets:
-          - grafana:3000
-
   - job_name: node-exporter-remote
     file_sd_configs:
       - files: