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/__init__.py

@@ -0,0 +1,6 @@
+from app.ml.features import (
+    build_price_features,
+    build_battery_features,
+)
+
+__all__ = ["build_price_features", "build_battery_features"]