Align default Replicate model to the versioned Kokoro model across UI, docs,
and server logic. Refactor TTS settings merging for audiobooks to ensure
consistent normalization. Improve Retry-After header handling for upstream
rate limits. Expand Replicate voice resolution to use model schemas when
available, with test coverage for custom and built-in models.
Update documentation to reflect new Replicate defaults, model selection,
and configuration guidance.
893f74f038