Everything OpenVoice does that we didn't, built from models we already run rather
than adopting theirs — plus MeloTTS itself so their exact V2 pairing is available.

Engines on /v1/audio/clone (`engine`, default `auto`):
  f5       today's in-context clone — timbre AND prosody, needs the transcript
  xtts     XTTS-v2 zero-shot from the clip, 17 languages, no transcript
  chain    base TTS -> seed-vc timbre transfer: OpenVoice's architecture with
           seed-vc in place of its tone-colour converter
  melotts  the same chain with MeloTTS as the base voice
`auto` picks a transcript-free engine when the profile has no transcript or the
target language differs from the reference — the case where F5 carries the wrong
accent across. GET /v1/audio/clone/engines reports what is installed.

The transcript is no longer a hard requirement: it is now checked per engine
instead of rejecting the request outright.

Multi-clip profiles: PATCH /v1/audio/voices/{name} takes add_clips/remove_clips,
and a clone prompts on the cleanest take. Scored on clipping, level and crest
factor — explicitly NOT loudest-wins, since a clipped take has the highest RMS of
the set and is the worst possible prompt. Single-clip profiles keep working.

Watermarking with AudioSeal (MIT), on by default, per-request or global override,
detection at POST /v1/audio/watermark/detect. Missing audioseal degrades to
unmarked audio with one warning, never an error.

MeloTTS runs in an isolated venv worker like parler-tts — its pins conflict with
this stack the same way. _RemoteParlerBackend generalised to _RemoteSpeechBackend
since both speak the same POST /speak contract.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY