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/tests/test_api/__init__.py

@@ -0,0 +1,3 @@
+from tests.conftest import sample_price_data, sample_battery_state
+
+__all__ = ["sample_price_data", "sample_battery_state"]