List voice conversion models

Convert the speaker identity of source audio to a target voice. Keeps the words, timing, and prosody of the source recording while re-rendering it in the target speaker's voice (zero-shot, via the Chatterbox S3Gen stack). Provide either `target_voice_id` (saved library voice) or `target_audio_base64` (6-10s of clean speech). Use `?sync=true` to block until audio is ready. Otherwise poll `GET /api/v1/conversion/progress` and `GET /api/v1/conversion/result`.