- Add Polygon.io API client for reliable historical data access
- Update bot to use Polygon instead of Yahoo Finance for charts
- Add POLYGON_API_KEY to config and environment example
- Polygon free tier: 5 API calls/minute, more reliable than Yahoo
- Fallback to FinViz chart URL when Polygon unavailable
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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
