Replace Yahoo Finance with Polygon.io for chart data

- 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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.env.example

@@ -11,6 +11,7 @@ COMMAND_PREFIX=!
 PRIMARY_TICKER=PYPL
 STOCK_API_PROVIDER=finnhub
 FINNHUB_API_KEY=your_finnhub_api_key_here
+POLYGON_API_KEY=your_polygon_api_key_here
 
 # Scheduling Configuration
 UPDATE_INTERVAL_HOURS=1