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/integrations/registry.ts

@@ -1,4 +1,5 @@
 import type { ComponentType } from "react";
+import { AlertmanagerAlertsWidget } from "../widgets/AlertmanagerAlertsWidget";
 import { BackupsWidget } from "../widgets/BackupsWidget";
 import { GrafanaLinkWidget } from "../widgets/GrafanaLinkWidget";
 import { JellyfinWidget } from "../widgets/JellyfinWidget";
@@ -42,6 +43,28 @@ export interface ServiceBinding {
 }
 
 export const SERVICE_REGISTRY: Record<string, ServiceBinding> = {
+	alertmanager: {
+		serviceType: "alertmanager",
+		name: "Alertmanager",
+		description: "Alertmanager alerts and status.",
+		widgets: [
+			{
+				kind: "active_alerts",
+				name: "Active alerts",
+				description: "Firing alerts summary from Alertmanager.",
+				refreshIntervalMs: 30_000,
+				defaultConfig: {},
+				configSchema: {
+					type: "object",
+					properties: {
+						severity_filter: { type: "string" },
+					},
+					required: [],
+				},
+				component: AlertmanagerAlertsWidget,
+			},
+		],
+	},
 	grafana: {
 		serviceType: "grafana",
 		name: "Grafana",