feat(observability): resolve services from registry, add health endpoints

Slice 2 of observability-service-registry. The monitoring router resolves
observability components from the service registry instead of env vars.

- routers/monitoring.py: removed _alertmanager_client/_webhook_client env
  readers + the get_settings import. Added _resolve_service_record(store,
  service_type, service_id?) -> ServiceRecord|None (requested instance with
  type+enabled checks, else first enabled instance), plus _base_url/_timeout/
  _auth_headers (Bearer from api_key)/_status_response helpers.
- /alerts + /alertmanager-status now take service_id? + Depends(store),
  resolve an alertmanager service, return graceful not-configured/
  unreachable payloads including service_id/name; status down-branches now
  include peers:[] + error (fixes prior type drift).
- NEW /grafana-status (probes /api/health) and /prometheus-status (probes
  /-/healthy then /api/v1/status/buildinfo) returning
  {up,version,service_id,name,error}.
- Webhook receiver is now log-only (dropped the outbound
  ALERTMANAGER_WEBHOOK_URL forward).
- tests: rewrote TestAlertmanager + TestAlertmanagerWebhook to mock
  _resolve_service_record/requests.get (not-configured via empty registry);
  added TestGrafanaStatus/TestPrometheusStatus and a TestResolveServiceRecord
  unit class covering service_id match/type-mismatch/disabled and first-
  enabled/none-enabled paths.

Orphaned config fields alertmanager_url/alertmanager_webhook_url and the
env-var removal land in Slice 5. ruff clean; 240 backend tests pass.

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

backend/tests/test_widgets.py

@@ -380,9 +380,7 @@ async def test_alertmanager_adapter_sends_bearer_token():
         config={"base_url": "http://am:9093"},
         secrets={"api_key": "tok"},
     )
-    payload = SimpleNamespace(
-        raise_for_status=lambda: None, json=lambda: {"status": "success", "data": []}
-    )
+    payload = SimpleNamespace(raise_for_status=lambda: None, json=lambda: {"status": "success", "data": []})
     with patch("media_library_viewer_api.widgets.sources.requests.get", return_value=payload) as mock_get:
         result = await adapter.fetch(service, "active_alerts", {})
     assert result["total"] == 0