When a clip has multiple speakers, pyannote's auto-detect sometimes collapses
them into a single "Speaker 1" — so the transcript merges turns and the dub
mixes voices. The diarization-consumption side is correct (overlap-weighted,
distinct Speaker N ids — pinned by a new test), so the collapse comes from
auto-detect itself.
Add an optional speaker-count hint (the reporter's own suggestion):
- backend: `/dub/transcribe-stream/{job_id}?num_speakers=N` (clamped 1–20;
None → auto-detect) threaded to `diar_pipe(audio, num_speakers=N)`. Omitted
entirely when unset so we don't depend on the kwarg in every pyannote build.
- frontend: `dubNumSpeakers` store field + a compact "Speakers" number input
in the dub panel (placeholder "Auto") + i18n; `transcribeStreamUrl` appends
the param; the SSE hook reads the hint at stream-open time.
Tests: tests/test_assign_speakers_from_diarization.py (multi-speaker split,
overlap weighting, label robustness, empty-result safety) +
dub.transcribeUrl.test.ts (param appended only for a positive int). Full
backend diarization + frontend suites pass; CJK i18n guard passes.
Does NOT close#274 — pending the reporter confirming that setting the count
resolves the collapse on their video (can't verify pyannote behaviour without
a CUDA box + the clip).
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
