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/tests/test_services.py

@@ -56,10 +56,11 @@ def client(tmp_path):
 # ---------------------------------------------------------------------------
 
 
-def test_registry_contains_five_service_types():
+def test_registry_contains_seven_service_types():
     assert set(SERVICE_DEFINITIONS) == {
         "grafana",
         "prometheus",
+        "alertmanager",
         "jellyfin",
         "jellyseerr",
         "nextcloud",
@@ -70,6 +71,7 @@ def test_registry_contains_five_service_types():
 def test_definitions_declare_widget_kinds():
     assert {wk.kind for wk in get_service_definition("grafana").widget_kinds} == {"link"}
     assert {wk.kind for wk in get_service_definition("prometheus").widget_kinds} == {"metric"}
+    assert {wk.kind for wk in get_service_definition("alertmanager").widget_kinds} == {"active_alerts"}
     assert {wk.kind for wk in get_service_definition("jellyfin").widget_kinds} == {"activity"}
     assert get_service_definition("nextcloud").widget_kinds == []
     assert {wk.kind for wk in get_service_definition("ssh_tasks").widget_kinds} == {"task_output"}
@@ -135,6 +137,7 @@ def test_list_service_types(client):
     assert response.status_code == 200
     types = {item["service_type"] for item in response.json()}
     assert types == {
+        "alertmanager",
         "grafana",
         "jellyfin",
         "jellyseerr",