Add matplotlib-based chart generation with Yahoo Finance data

- Add chart_generator.py with price and candlestick chart support
- Implement Yahoo Finance candle data fetching for free historical data
- Update bot to generate and attach charts to stock embeds
- Add matplotlib dependency to requirements.txt
- Configure dual API approach: Finnhub for quotes, Yahoo for charts

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

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

requirements.txt

@@ -5,3 +5,4 @@ APScheduler==3.10.4
 python-dotenv==1.0.0
 pytz==2023.3
 requests==2.31.0
+matplotlib==3.8.2