feat: add CAMB AI as a TTS and voice cloning provider

[neilruaro-camb]

Summary

• Adds CAMB AI as an alternative TTS and voice cloning provider alongside Fish Speech
• Controlled via TTS_PROVIDER environment variable (fish or camb)
• Voice cloning uses CAMB's cloud API (no local model server needed)
• Streaming TTS via MARS speech models (mars-flash for ~150ms latency, mars-pro for higher quality)
• Backward compatible: defaults to Fish Speech when TTS_PROVIDER is not set

About CAMB AI

CAMB AI is a voice AI and localization platform trusted by brands like the Premier League, the NBA, NASCAR, and the Australian Open. We'd love for CAMB AI to be available as a TTS and voice cloning option in Duix Avatar.

Usage

1. Set environment variables:
   • TTS_PROVIDER=camb
   • CAMB_API_KEY=your_key (from https://studio.camb.ai)
2. Run the app normally
3. Voice training will use CAMB cloud voice cloning
4. Audio generation will use CAMB streaming TTS

Files changed

• src/main/api/camb-tts.js - New CAMB API wrapper (TTS streaming, voice cloning, list voices)
• src/main/config/config.js - Added ttsProvider toggle and cambConfig
• src/main/service/voice.js - Branch on provider for voice training and audio generation
• src/main/dao/voice.js - Added insertCamb() for CAMB voice records
• src/main/db/sql.js - DB migration adding camb_voice_id column