Add Speechmatics transcription provider

[zhengjynicolas]

Summary

• add a Speechmatics Batch transcription backend available through --api speechmatics
• wire the provider into the CLI and document the required Speechmatics environment variables
• add mocked unit tests for job creation, polling, transcript retrieval, and CLI routing

Validation

• /tmp/sapat-speechmatics-venv/bin/python -m unittest discover -s tests -v
• /tmp/sapat-speechmatics-venv/bin/python -m compileall src tests
• /tmp/sapat-speechmatics-venv/bin/python -m sapat.script --help
• git diff --check

This is a companion implementation for daytonaio/content#13.