SQL: Running Total Per Customer (SUM OVER)

> This problem is paper-only - see explanation for the canonical SQL solution. Given `Orders(id, customer_id, order_date, total)`, return each row plus a `running_total` column that is the cumulative spend for that customer up to and including the current row, ordered by `(order_date, id)` within each customer. Sort the output by customer_id ascending, order_date ascending, id ascending. **For the auto-grader:** return `[{ id, customer_id, order_date, total, running_total }]`.