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>

stock_api/yahoo.py

@@ -1,6 +1,6 @@
 import yfinance as yf
 import logging
-from typing import Optional, Dict, Any
+from typing import Optional, Dict, Any, List
 from .base import StockAPIBase
 
 
@@ -83,3 +83,46 @@ class YahooFinanceAPI(StockAPIBase):
         except Exception as e:
             logger.error(f"Yahoo Finance API unavailable: {e}")
             return False
+
+    def get_candles(self, ticker: str, days: int = 30, resolution: str = 'D') -> Optional[Dict[str, List]]:
+        """
+        Retrieve historical candlestick data from Yahoo Finance.
+
+        Args:
+            ticker: Stock ticker symbol
+            days: Number of days of historical data to fetch
+            resolution: Candle resolution (ignored, always daily for Yahoo Finance)
+
+        Returns:
+            Dictionary with OHLCV data or None if unavailable
+        """
+        try:
+            stock = yf.Ticker(ticker)
+
+            # Fetch historical data
+            hist = stock.history(period=f"{days}d")
+
+            if hist.empty:
+                logger.warning(f"No candle data available for ticker: {ticker}")
+                return None
+
+            # Convert to the format expected by chart generator
+            timestamps = [int(date.timestamp()) for date in hist.index]
+            opens = hist['Open'].tolist()
+            highs = hist['High'].tolist()
+            lows = hist['Low'].tolist()
+            closes = hist['Close'].tolist()
+            volumes = hist['Volume'].tolist()
+
+            return {
+                'timestamps': timestamps,
+                'open': opens,
+                'high': highs,
+                'low': lows,
+                'close': closes,
+                'volume': volumes
+            }
+
+        except Exception as e:
+            logger.error(f"Error fetching candle data for {ticker}: {e}")
+            return None