feat(observability): service discovery, health cards, alertmanager widget

Slice 3 of observability-service-registry (frontend). The Observability
page discovers Grafana from the service registry instead of env vars,
adds Grafana + Prometheus health cards, and ships an alertmanager
active_alerts dashboard widget.

- types: added GrafanaStatus + PrometheusStatus; added optional
  service_id/error to AlertmanagerStatus.
- api/client.ts + hooks/useObservability.ts: fetchGrafanaStatus,
  fetchPrometheusStatus, useGrafanaStatus, usePrometheusStatus.
- widgets/AlertmanagerAlertsWidget.tsx (new): presentational widget
  consuming the active_alerts summary shape (total/by_severity/alerts);
  exported from widgets/index.ts.
- integrations/registry.ts: alertmanager binding (active_alerts kind,
  30s refresh, optional severity_filter); registry.test.ts updated to
  6 service types incl alertmanager + a resolve test.
- components/ObservabilityPage.tsx: removed
  import.meta.env.VITE_GRAFANA_URL; derive GRAFANA_BASE_URL from the
  first enabled grafana service via useServiceInstances("grafana");
  added Grafana + Prometheus HealthCards (up/not-configured/unreachable)
  with QueryError retry blocks; machine dashboard shows a "No Grafana
  service configured" empty-state linking to /services when none is set.

npm run build (tsc -b + vite) clean; 0 lint errors; 72 frontend tests
pass. Reviewed fresh-context (read-only): no blockers.

frontend/src/hooks/useObservability.ts

@@ -2,6 +2,8 @@ import { useQuery } from "@tanstack/react-query";
 import {
 	fetchAlertmanagerAlerts,
 	fetchAlertmanagerStatus,
+	fetchGrafanaStatus,
+	fetchPrometheusStatus,
 	fetchPrometheusTargets,
 	fetchMonitoringMachines,
 } from "../api/client";
@@ -26,6 +28,26 @@ export function useAlertmanagerStatus() {
 	});
 }
 
+export function useGrafanaStatus() {
+	return useQuery({
+		queryKey: ["observability", "grafana-status"],
+		queryFn: fetchGrafanaStatus,
+		retry: 2,
+		staleTime: 10_000,
+		refetchInterval: 30_000,
+	});
+}
+
+export function usePrometheusStatus() {
+	return useQuery({
+		queryKey: ["observability", "prometheus-status"],
+		queryFn: fetchPrometheusStatus,
+		retry: 2,
+		staleTime: 10_000,
+		refetchInterval: 30_000,
+	});
+}
+
 export function usePrometheusTargets() {
 	return useQuery({
 		queryKey: ["observability", "prometheus-targets"],