The finance team wants to understand what portion of paid-order revenue comes from premium customers.
The orders table records each order's customer segment (premium or free) and its amount in amount_usd. Prepare a single summary showing total revenue from paid orders, the portion earned from premium customers, and the premium portion as a fraction of paid revenue (for example, 0.5 means half).
If there are no paid orders, premium_share must be 0 rather than NULL or an error.
Ordering: Return a single row; sorting is not required.