CrisperWhisper does verbatim ASR with precise word timestamps, but its custom
generation config crashes on the server's transformers 5.x. Run it in its own
venv worker (pinned torch 2.5.1+cu124 + transformers 4.46.3 — last pre-5.x with
working Whisper word timestamps and a py3.13 tokenizers wheel):
- codai/api/crisperwhisper_worker.py + tools/crisperwhisper_service.py
  (return_timestamps="word" + CrisperWhisper pause redistribution).
- requirements-crisperwhisper.txt.
- stt_backends.py: `crisperwhisper` is its own family -> _RemoteCrisperWhisperBackend
  (HTTP to the worker); generic `whisper-hf` still runs in the shared venv
  (text-only default + graceful timestamp fallback); HF-ASR path now converts any
  container (mp4/m4a/webm) to 16k mono wav first and uses model-correct
  return_timestamps (Whisper True/"word", CTC "chunk"/"word").
- transcriptions.py: crisperwhisper dispatched as a GPU worker.
- manager skip-download + admin backend option for whisper-hf/crisperwhisper.

Verified: mp4 -> verbatim text + per-word timestamps, HTTP 200, on the 3090.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R6iugoNeEgt9PkajKrFyq4