Adds /api/costs/tts endpoint backed by get_tts_providers() in cost_db,
surfaces a Costs link in the control panel header, and small polish
to the costs page HTML/JS.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
- Add check_same_thread=False for background thread access
- Add standalone timestamp index for non-session queries
- Join to sessions table for consistent period filtering in get_models/categories/expensive_calls
- Wrap record imports in try/except with .get() for timestamp
- Add avg_cost_per_session to get_summary()
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>