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