Add comprehensive error handling for TTS synthesis failures:
Backend:
- Add TTS_FAILED status to JobStatus enum for failed synthesis jobs
- Add TTSSynthesisError exception with cue index and context tracking
- Improve null-safe error handling in Gemini TTS response parsing
- Add _synthesize_cue_with_retry() with exponential backoff (3 attempts)
- Enhanced error logging with text preview and model context
Frontend:
- Add TTS_FAILED status styling (red badge) in StatusBadge component
- Add tts_failed to JobStatus TypeScript type
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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