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/core/constants.py

@@ -0,0 +1,19 @@
+from datetime import timedelta
+
+DEFAULT_BATTERY_CAPACITY_MWH = 100.0
+DEFAULT_CHARGE_RATE_MW = 50.0
+DEFAULT_DISCHARGE_RATE_MW = 50.0
+DEFAULT_EFFICIENCY = 0.90
+
+DEFAULT_HEARTBEAT_INTERVAL = 30
+WS_HEARTBEAT_INTERVAL = 30
+
+PRICE_DATA_REGIONS = ["FR", "BE", "DE", "NL", "UK"]
+
+DEFAULT_DATA_LIMIT = 1000
+
+BACKTEST_RESULT_TIMEOUT = timedelta(hours=1)
+
+TRAINING_RESULT_TIMEOUT = timedelta(hours=24)
+
+MAX_BACKTEST_TRADES = 10000