Probe Fireworks

Prove the pasted Fireworks key actually works, on demand.

Frederick, 2026-09-29 (via handoff rooster-transcription-efficiency- 2026-09-29.md): seat 4 (Nemotron) fails 68% of transcription reads on NVIDIA's free tier. Fireworks is the trial candidate; this route is what ApiKeyTestPanel.tsx's "Test" button calls right after a save -- the same "save then verify" shape /api/system/wise/probe gives Wise's own card. GET /models on Fireworks' inference host, never a priced completion. PASS/FAIL ONLY, per the build task: the panel shows "Connected" or "Fireworks rejected this key", nothing about which model the trial will use (that is the trial tool's own job, not a credential probe's).

Never raises on a bad key -- models.probe_fireworks_key is itself non-raising; this only records the verdict and translates it into what the panel shows.