Gemini TTS allows 10 RPM; with concurrency=8 the rate limit is hit quickly.
The previous backoff (1-3s) was far too short — the API returns retryDelay ~37s.
Both synthesize_cue_task (Celery retry countdown) and GeminiTTSService
(_synthesize_cue_with_retry sleep) now parse the retryDelay from the 429
error message and use it (+ 5s buffer) instead of the exponential guess.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>