fix: add missing logger import in plotly_charts module

The correlation heatmap function referenced logger without importing it,
causing NameError at runtime. Added logging import and module-level logger
instance to resolve the issue.

backend/visualizations/plotly_charts.py

@@ -14,9 +14,12 @@ from plotly.subplots import make_subplots
 import pandas as pd
 import numpy as np
 import os
+import logging
 from typing import Dict, List, Any, Optional
 from decimal import Decimal
 
+logger = logging.getLogger(__name__)
+
 
 def get_plotly_template() -> str:
     """Get Plotly template that supports light/dark mode.