Compute the median amount_usd per customer WITHOUT using PERCENTILE_CONT, MEDIAN, or any percentile-flavored aggregate. Use only window functions, basic aggregates, and CASE.
The median, for a customer's n orders sorted by amount: when n is odd, the amount at position (n+1)/2; when n is even, the average of the amounts at positions n/2 and n/2 + 1.
Sort by customer_id ascending.