TTS provider: add support for OpenAI TTS API #342

Add support for the OpenAI TTS API (/v1/audio/speech): https://developers.openai.com/api/docs/guides/text-to-speech

This has become a de facto standard interface for TTS, implemented by both local/self-hosted servers and cloud providers. Supporting it would let users plug in many different TTS backends with minimal integration effort, instead of writing a custom integration per service. Examples:

Local / self-hosted:

• Kokoro-FastAPI — wraps Kokoro-82M
• pocket-tts-openapi — wraps Kyutai's Pocket TTS
• Local-TTS-Service — multi-engine local TTS server
• Chatterbox-TTS-Server — wraps Resemble AI's Chatterbox, runs on CPU/NVIDIA/AMD
• AllTalk TTS — multi-backend local TTS server
• openai-edge-tts — free local wrapper around Microsoft Edge's TTS voices

Cloud:

• OpenRouter — routes to multiple TTS models via one OpenAI-shaped endpoint
• TTS.ai — OpenAI-compatible endpoint giving access to 20+ hosted models
• LiteLLM — proxy/SDK that normalizes many TTS providers (including OpenAI itself) behind this same schema