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/api/__init__.py

@@ -0,0 +1,3 @@
+from app.api.routes import dashboard, backtest, models, trading, settings
+
+__all__ = ["dashboard", "backtest", "models", "trading", "settings"]