Time series forecasting with Prophet

Build a time series forecasting model using Facebook Prophet. Steps: 1. Prepare historical sales data with daily granularity. 2. Add custom seasonality for Black Friday and holiday peaks. 3. Include external regressors (marketing spend, weather). 4. Generate 90-day forecast with uncertainty intervals. 5. Validate model using cross-validation and MAPE metric. Visualize actual vs predicted with interactive Plotly charts.