feat(tts): add Doubao TTS 2.0 (Volcengine Seed-TTS 2.0) provider

[wyuc]

Description

Add Doubao TTS 2.0 as a new TTS provider, powered by Volcengine's Seed-TTS 2.0 API.

Features

• 17 built-in voices (14 Chinese Mandarin, 3 English US)
• Speed control (0.5x - 2.0x)
• Streaming response parsing for Volcengine's chunked JSON format
• Rate-limit error detection (codes 45000000 / 45000292)
• Server-side config via TTS_DOUBAO_API_KEY env var or server-providers.yml
• API key format: appId:accessKey (both from Volcengine console)

API Reference

• Volcengine Seed-TTS 2.0 Documentation