[MEDIUM] Build ARIMA Baseline Forecasting Model in src/models/arima_model.py

[ssndb134]

Description:
Create a baseline ARIMA or Auto-ARIMA model for forecasting ticket counts.

Acceptance Criteria:

• Save fitted ARIMA model to models/arima.pkl.
• Save residual plot to plots/arima_residuals.png.
• Handle missing or non-stationary data gracefully.