[Alpha] Implement stacked LSTM for predicted alpha in event market

[CYX22222003]

Adds a new LSTM-based alpha model with test coverage.

Event LSTM Alpha

• Implements EventLstmAlpha in alpha/EventLstmAlpha.py.
• Loads a pre-trained stacked LSTM and generates predictions from the DataFrame's "close" prices.
• Predictions are appended as a lstm_pred column.

Testing

• Adds TestEventLstmAlpha in tests/test_alpha.py using synthetic OHLCV data.
• Tests verify the prediction column exists, NaNs are handled correctly, and predictions are within the expected range.