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.yml

@@ -27,7 +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}
+      ALERTMANAGER_WEBHOOK_URL: ${ALERTMANAGER_WEBHOOK_URL:-}
     volumes:
       - ${BACKEND_CACHE_DIR:-./backend-cache}:/app/backend/.cache
     restart: unless-stopped
@@ -69,6 +69,7 @@ services:
         VITE_OIDC_REDIRECT_URI: ${VITE_OIDC_REDIRECT_URI:?set VITE_OIDC_REDIRECT_URI}
         VITE_OIDC_POST_LOGOUT_REDIRECT_URI: ${VITE_OIDC_POST_LOGOUT_REDIRECT_URI:?set VITE_OIDC_POST_LOGOUT_REDIRECT_URI}
         VITE_DEV_API_PROXY_TARGET: ${VITE_DEV_API_PROXY_TARGET:-http://backend:8000}
+        VITE_GRAFANA_URL: ${VITE_GRAFANA_URL:-https://grafana.example.com}
         VITE_APP_VERSION: ${APP_VERSION:-0.1.0}
         VITE_APP_BUILD_INFO: ${APP_BUILD_INFO:-dev}
     depends_on: