Add comprehensive design for moving price data downloads from
synchronous API endpoint to background worker thread.
Key changes:
- Fast API response (<1s) by deferring download to worker
- New job status "downloading_data" for visibility
- Graceful rate limit handling with warnings
- Enhanced logging for dev mode monitoring
- Backwards compatible API changes
Resolves API timeout issue when downloading missing price data.
Co-Authored-By: Claude <noreply@anthropic.com>
