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.
Developer
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