feat(observability): add alertmanager service type and widget

Slice 1 of observability-service-registry. Alertmanager becomes a
first-class service-registry type, mirroring grafana/prometheus.

- integrations/alertmanager.py (new): AlertmanagerConfig
  (base_url, timeout_seconds), AlertmanagerAlertsWidgetConfig (optional
  severity_filter), shared summarize_alerts() helper, and DEFINITION
  (service_type "alertmanager", secret api_key, widget "active_alerts").
- integrations/registry.py: register ALERTMANAGER (7 types now).
- widgets/sources.py: AlertmanagerWidgetSource fetches
  {base_url}/api/v1/alerts, sends optional Bearer token from the api_key
  secret, applies optional severity_filter, and summarizes via the shared
  helper; registered in SERVICE_ADAPTERS.
- routers/monitoring.py: _summary_from_alerts delegates to the shared
  summarize_alerts (behavior unchanged).
- tests: registry now 7 types; /api/services/types lists alertmanager;
  4 new adapter tests (summarize, severity filter, bearer token, missing
  service).

Backend-only slice; the frontend active_alerts widget binding lands in a
later slice. ruff clean; 228 backend tests pass.

Reviewed fresh-context (read-only): no blockers.

backend/src/media_library_viewer_api/widgets/sources.py

@@ -22,6 +22,7 @@ from media_library_viewer_api.domain.dashboard import (
     _map_sessions_to_activity_rows,
     build_backup_dashboard_summary,
 )
+from media_library_viewer_api.integrations.alertmanager import summarize_alerts
 from media_library_viewer_api.services.settings_store import SettingsStore, get_settings_store
 from media_library_viewer_api.services.task_runner import run_saved_task
 
@@ -151,6 +152,43 @@ class PrometheusWidgetSource:
             return {"error": f"Prometheus query failed: {exc}"}
 
 
+class AlertmanagerWidgetSource:
+    """Fetch firing alerts from an Alertmanager service and summarize them."""
+
+    async def fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) -> dict[str, Any]:
+        try:
+            if service is None:
+                return {"error": "Alertmanager widget is missing its service"}
+            base_url = str(service.config.get("base_url") or "").rstrip("/")
+            timeout = int(service.config.get("timeout_seconds") or 5)
+            severity_filter = config.get("severity_filter") or None
+            headers: dict[str, str] = {}
+            api_key = str(service.secrets.get("api_key") or "")
+            if api_key:
+                headers["Authorization"] = f"Bearer {api_key}"
+            response = await asyncio.wait_for(
+                asyncio.to_thread(
+                    requests.get,
+                    f"{base_url}/api/v1/alerts",
+                    headers=headers,
+                    timeout=timeout,
+                ),
+                timeout=timeout,
+            )
+            response.raise_for_status()
+            payload = response.json()
+            alerts = payload.get("data", []) if isinstance(payload, dict) else []
+            return summarize_alerts(alerts, severity_filter=severity_filter)
+        except asyncio.TimeoutError:
+            return {"error": "Widget data fetch timed out"}
+        except requests.RequestException as exc:
+            logger.exception("alertmanager adapter failed")
+            return {"error": f"Alertmanager query failed: {exc}"}
+        except Exception as exc:
+            logger.exception("alertmanager adapter failed")
+            return {"error": f"Alertmanager query failed: {exc}"}
+
+
 class JellyfinWidgetSource:
     """Fetch Jellyfin sessions and map them to activity rows."""
 
@@ -234,6 +272,7 @@ def _record_timeout(service: ServiceRecord | None, config: dict[str, Any], timeo
 SERVICE_ADAPTERS: dict[str, WidgetSource] = {
     "grafana": GrafanaWidgetSource(),
     "prometheus": PrometheusWidgetSource(),
+    "alertmanager": AlertmanagerWidgetSource(),
     "jellyfin": JellyfinWidgetSource(),
     "ssh_tasks": SshTaskWidgetSource(),
 }