Create risk manager module (risk_manager.py)

[bryanssss]

Develop a robust risk management module for position sizing and loss mitigation strategies (risk_manager.py).

• Implement stop-loss and take-profit logic
• Limit max drawdown and max position size per trade
• Enable real-time portfolio risk calculations
• Write basic test cases

Acceptance Criteria:

• Position sizing and stop-loss logic functional
• Coverage for edge cases (low liquidity, high volatility)
• Usage documented in README