Add Symbl.ai transcription provider

[jtc268]

Adds a Symbl.ai Async Audio API transcription provider for Sapat.

Scope:

• Adds --api symbl CLI routing.
• Supports SYMBL_ACCESS_TOKEN or SYMBL_APP_ID / SYMBL_APP_SECRET token generation.
• Submits MP3 audio to Symbl's async audio endpoint, polls the job, and reads conversation messages into Sapat's existing transcript output path.
• Documents Symbl env vars in README and .env.example.
• Adds mocked unit coverage for submission, polling, transcript retrieval, token generation, and failed jobs.

Validation:

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

No secrets or media files are included.

[nibzard]

Thank you for this contribution! Your implementation was reviewed as part of a batch review of all incoming provider PRs.

We've decided to take a different approach: instead of merging individual provider PRs (which all targeted a different code architecture than what exists on main), we've refactored the entire codebase into a proper provider plugin architecture with auto-discovery and shared mixins. Your provider's API integration patterns were used as reference when building the Symbl.ai provider in the new architecture.

The refactor is on branch feat/provider-plugin-architecture and includes 29 providers, 176 tests, and a clean plugin system. We appreciate your contribution — it helped shape the final design. Closing this in favor of the unified approach.