- Split preWarmBusy from ttsBusy so user requests are never blocked by pre-warm
- /tts endpoint waits up to 6s for pre-warm synthesis then proceeds
- Pre-warm skips articles when user is actively using TTS
- Fix text format to match Brief exactly (remove substring(60), fix source fallback)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>