Opening Power Backend Engine Built

Full strategy engine at backend/app/services/strategies/opening_power.py (750+ lines). State detector, bar classifier (elephant/tail/small), day simulator with entry/adds/trailing stops/profit taking. Backtester uses unified data provider (Alpaca→yfinance). Scanner for pre-market screening. API: POST /api/strategies/opening-power/backtest and /scan. All major stocks currently in narrow state.