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/integrations/registry.py

@@ -6,6 +6,7 @@ There is no runtime plugin loading.
 
 from __future__ import annotations
 
+from media_library_viewer_api.integrations.alertmanager import DEFINITION as ALERTMANAGER
 from media_library_viewer_api.integrations.base import ServiceDefinition, WidgetKind
 from media_library_viewer_api.integrations.grafana import DEFINITION as GRAFANA
 from media_library_viewer_api.integrations.jellyfin import DEFINITION as JELLYFIN
@@ -17,6 +18,7 @@ from media_library_viewer_api.integrations.ssh_tasks import DEFINITION as SSH_TA
 SERVICE_DEFINITIONS: dict[str, ServiceDefinition] = {
     GRAFANA.service_type: GRAFANA,
     PROMETHEUS.service_type: PROMETHEUS,
+    ALERTMANAGER.service_type: ALERTMANAGER,
     JELLYFIN.service_type: JELLYFIN,
     JELLYSEERR.service_type: JELLYSEERR,
     NEXTCLOUD.service_type: NEXTCLOUD,