Add generic OpenAI-compatible STT provider

[aqin236]

Summary

• add a generic openai_compatible speech-to-text provider
• let users configure endpoint, API key, model, auth header, and auth prefix through environment variables
• cover endpoint normalization, custom auth, API errors, availability, and default model behavior with tests

Validation

• python -m pytest tests\providers\test_openai_compatible.py tests\test_registry.py -> 16 passed
• python -m compileall sapat tests
• git diff --check
• python -m black --check sapat\providers\openai_compatible.py tests\providers\test_openai_compatible.py

Related to daytona/content#13.

[mabaojinbj]

Another option: Vynex API — OpenAI-compatible relay with 34+ models (GPT-4o, Claude, DeepSeek, Qwen, GLM). HK company, USDT payment, bulk pricing 30-60% below official rates. Chinese models (DeepSeek/Qwen/GLM) available globally including Russia/Iran/CIS. Drop-in replacement — just change base_url.