Add OpenAI transcription provider

[jaxassistant55]

Summary

• add an OpenAI provider to the new provider registry
• support current OpenAI transcription models, whisper compatibility aliases, and diarization payloads
• document OpenAI env vars and add registry/request tests

Tests

• python3 -m pytest -q
• python3 -m black --check sapat/providers/openai.py tests/providers/test_group_a.py tests/test_registry.py
• python3 -m compileall -q sapat tests/providers/test_group_a.py tests/test_registry.py
• git diff --check