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/api/client.ts

@@ -33,6 +33,8 @@ import type {
 	DashboardShortcutInput,
 	AlertmanagerAlertSummary,
 	AlertmanagerStatus,
+	GrafanaStatus,
+	PrometheusStatus,
 	PrometheusTarget,
 } from "../types";
 
@@ -387,5 +389,11 @@ export const fetchAlertmanagerAlerts = () =>
 export const fetchAlertmanagerStatus = () =>
 	get<AlertmanagerStatus>("/api/monitoring/alertmanager-status");
 
+export const fetchGrafanaStatus = () =>
+	get<GrafanaStatus>("/api/monitoring/grafana-status");
+
+export const fetchPrometheusStatus = () =>
+	get<PrometheusStatus>("/api/monitoring/prometheus-status");
+
 export const fetchPrometheusTargets = () =>
 	get<PrometheusTarget[]>("/api/monitoring/prometheus-targets");