Add FastAPI backend for energy trading system

Implements FastAPI backend with ML model support for energy trading,
including price prediction models and RL-based battery trading policy.
Features dashboard, trading, backtest, and settings API routes with
WebSocket support for real-time updates.

backend/app/ml/features/lag_features.py

@@ -0,0 +1,14 @@
+from typing import List
+import pandas as pd
+
+
+def add_lag_features(df: pd.DataFrame, col: str, lags: List[int]) -> pd.DataFrame:
+    result = df.copy()
+
+    for lag in lags:
+        result[f"{col}_lag_{lag}"] = result[col].shift(lag)
+
+    return result
+
+
+__all__ = ["add_lag_features"]