Transition all TTS-related logic, types, and UI to use the new providerRef/providerType model in place of legacy ttsProvider fields. Introduce a centralized tts-provider-policy module to encapsulate provider/model capability checks, default value resolution, and compatibility logic. Update all API routes, contexts, hooks, components, and tests to use providerRef and providerType, ensuring consistent handling of built-in and shared TTS providers. Remove legacy defaultTtsModel config in favor of per-provider defaults and shared provider admin control. Add the showAllProviderModels runtime flag to restrict users to provider default models when desired.
BREAKING CHANGE: ttsProvider fields are replaced by providerRef/providerType throughout the codebase; defaultTtsModel config is removed in favor of per-provider defaults.
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