Convert audio and video audio tracks to mono 16 kHz WAV with FFmpeg,
then submit them to OpenRouter's MAI-Transcribe-2 endpoint. Read the API
key from the environment and print the transcript to stdout for easy
redirection. Store request payloads in temporary files to avoid shell
argument size limits, and document dependencies and usage.
Test Plan:
- bash -n transcribe and shellcheck transcribe passed.
- All eight documented formats passed local FFmpeg conversion and mocked
API request/response checks, including WAV rate and channel validation.
- git diff --cached --check passed.
- No live API request was made.
