Dependency on grafana-metric-gateway: the prometheus service now sources via
Grafana /api/ds/query (no direct Prom endpoint). Test routine changes from
GET /api/v1/query?query=up to POST {grafana_url}/api/ds/query with api_key +
datasource_uid, expr 'up'.
On-demand per-service-type credential tester for the add/edit dialog. Model on
test_machine_ssh. POST /api/services/test dispatching to per-type routines
(qBit login+maindata, Prom instant query, Jellyfin /Users, etc.) returning
{ok, detail, evidence}. Frontend Test button + gate Create/Save on pass with
'Save anyway' override. Resolves#3 (qBit login failures visible in UI not
logs) and complements #1 (validation surfacing, already shipped).
Developer