Eliminate the deprecated `/api/tts` route and its client usage, consolidating TTS generation logic under chapter and segment endpoints. Update client API utilities to remove direct TTS buffer fetching and improve error propagation for chapter generation failures. Extend TTS segment error metadata to include upstream rate limit and retry details, and standardize error typing in shared types. Adjust tests and helpers to reflect the new TTS mocking approach and endpoint structure. This streamlines TTS API surface and enhances error reporting consistency.
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