feat: add Venice transcription provider

[Da7-Tech]

Summary

• add a Venice transcription provider using the multipart audio transcription endpoint
• wire --api venice into the CLI provider selection
• document the required Venice environment variables and usage example
• add focused unit coverage for the request payload and missing API key handling

Verification

• PYTHONPATH=src python3 -m unittest tests.test_venice -v
• python3 -m py_compile src/sapat/script.py src/sapat/transcription/venice.py tests/test_venice.py
• git diff --check

No secrets are included. The provider reads credentials from VENICE_API_KEY.

[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 Venice 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.