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/routers/monitoring.py

@@ -33,32 +33,9 @@ def _webhook_client() -> Any:
 
 def _summary_from_alerts(alerts: list[dict[str, Any]]) -> dict[str, Any]:
     """Build a UI-friendly summary from Alertmanager /api/v1/alerts payload."""
-    by_severity: dict[str, int] = {}
-    open_alerts: list[dict[str, Any]] = []
-    for alert in alerts:
-        labels = alert.get("labels") or {}
-        annotations = alert.get("annotations") or {}
-        severity = labels.get("severity", "unknown")
-        by_severity[severity] = by_severity.get(severity, 0) + 1
-        open_alerts.append(
-            {
-                "name": labels.get("alertname", "unknown"),
-                "severity": severity,
-                "category": labels.get("category", ""),
-                "job_name": labels.get("job_name", labels.get("job", "")),
-                "summary": annotations.get("summary", ""),
-                "description": annotations.get("description", ""),
-                "active_since": alert.get("startsAt"),
-                "state": alert.get("status", "firing"),
-                "labels": labels,
-            }
-        )
-    open_alerts.sort(key=lambda a: (a["severity"] not in {"critical", "warning"}, a["severity"], a["name"]))
-    return {
-        "total": len(open_alerts),
-        "by_severity": by_severity,
-        "alerts": open_alerts[:50],
-    }
+    from media_library_viewer_api.integrations.alertmanager import summarize_alerts
+
+    return summarize_alerts(alerts)
 
 
 router = APIRouter(prefix="/api/monitoring", tags=["monitoring"])