Build a Sales Report

Create a comprehensive sales report DataFrame showing per-product metrics.

Available Data:

DataFrame df with columns: date, product, quantity, price, region

Requirements:

• For each product, calculate:
• - total_qty: sum of quantity
• - total_revenue: sum of (quantity * price)
• - avg_qty_per_sale: mean of quantity
• Result: DataFrame with product as index, columns: total_qty, total_revenue, avg_qty_per_sale
• Sort by total_revenue descending