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/requirements.txt

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+fastapi>=0.104.0
+uvicorn[standard]>=0.24.0
+pydantic>=2.4.0
+pydantic-settings>=2.0.0
+pandas>=2.1.0
+numpy>=1.24.0
+pyarrow>=14.0.0
+xgboost>=2.0.0
+scikit-learn>=1.3.0
+gymnasium>=0.29.0
+stable-baselines3>=2.0.0
+celery>=5.3.0
+redis>=5.0.0
+websockets>=12.0.0
+sqlalchemy>=2.0.0
+alembic>=1.12.0
+python-multipart>=0.0.6
+python-jose[cryptography]>=3.3.0
+python-dotenv>=1.0.0
+loguru>=0.7.0
+pytest>=7.4.0
+pytest-asyncio>=0.21.0
+httpx>=0.25.0