Add Cloudflare Workers AI transcription provider

[aclanot]

Summary

• add a CloudflareTranscription provider for Cloudflare Workers AI Whisper
• expose the provider through sapat --api cloudflare
• document the required Cloudflare environment variables and update the Python version floor to match existing dependencies
• add mocked unittest coverage for request construction, binary upload, response parsing, and missing-token errors

Validation

• python -m unittest discover -s tests
• python -m compileall src tests
• uv run --with-editable . python -m sapat.script --help
• git diff --check

This is a companion implementation for the Daytona content bounty guide around running a Cloudflare-backed Sapat transcription workflow in a reproducible workspace.