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.

frontend/Dockerfile

@@ -15,6 +15,7 @@ ARG VITE_OIDC_SCOPE=openid profile email
 ARG VITE_OIDC_REDIRECT_URI=
 ARG VITE_OIDC_POST_LOGOUT_REDIRECT_URI=
 ARG VITE_DEV_API_PROXY_TARGET=http://backend:8000
+ARG VITE_GRAFANA_URL=https://grafana.example.com
 ARG VITE_APP_VERSION=0.1.0
 ARG VITE_APP_BUILD_INFO=dev
 
@@ -26,6 +27,7 @@ ENV VITE_API_URL=${VITE_API_URL} \
     VITE_OIDC_REDIRECT_URI=${VITE_OIDC_REDIRECT_URI} \
     VITE_OIDC_POST_LOGOUT_REDIRECT_URI=${VITE_OIDC_POST_LOGOUT_REDIRECT_URI} \
     VITE_DEV_API_PROXY_TARGET=${VITE_DEV_API_PROXY_TARGET} \
+    VITE_GRAFANA_URL=${VITE_GRAFANA_URL} \
     VITE_APP_VERSION=${VITE_APP_VERSION} \
     VITE_APP_BUILD_INFO=${VITE_APP_BUILD_INFO}
 
@@ -50,6 +52,7 @@ COPY frontend/ ./
 ENV VITE_API_URL=/api \
     VITE_OIDC_ENABLED=false \
     VITE_DEV_API_PROXY_TARGET=http://backend:8000 \
+    VITE_GRAFANA_URL=http://localhost:3000 \
     VITE_APP_VERSION=0.1.0 \
     VITE_APP_BUILD_INFO=dev