chore(config): remove dead GRAFANA_URL and wire VITE_GRAFANA_URL

- Remove GRAFANA_URL from backend environment (backend never consumed it).
- Add VITE_GRAFANA_URL to frontend build-args (prod), dev environment, and
  frontend/Dockerfile ARG/ENV so Grafana deep-links resolve correctly.
- Add ALERTMANAGER_WEBHOOK_URL to backend environment so the documented
  alert-forwarding feature is reachable from compose.
- Document VITE_GRAFANA_URL in .env.example.

docker-compose.dev.yml

@@ -16,7 +16,7 @@ services:
       SSH_KNOWN_HOSTS_PATH: /app/backend/.cache/known_hosts
       PROMETHEUS_FILE_SD_DIR: /app/backend/.cache/prometheus-file-sd
       ALERTMANAGER_URL: ${ALERTMANAGER_URL:-http://alertmanager:9093}
-      GRAFANA_URL: ${GRAFANA_URL:-http://grafana:3000}
+      ALERTMANAGER_WEBHOOK_URL: ${ALERTMANAGER_WEBHOOK_URL:-}
     ports:
       - "8000:8000"
     volumes:
@@ -37,6 +37,7 @@ services:
       VITE_API_URL: "/api"
       VITE_OIDC_ENABLED: "false"
       VITE_DEV_API_PROXY_TARGET: "http://backend:8000"
+      VITE_GRAFANA_URL: "http://localhost:3000"
     ports:
       - "5173:5173"
     volumes: