Returns the per-line returnable breakdown for an order: how many units
were ordered, how many have already been requested for return, and how
many remain returnable — plus the stable line_id to echo back as
line_id when you file a return (POST /returns).

Each line also lists the return requests already filed against it
(returns[]), so you can correlate by return number without a second
lookup.

The # prefix on the order number is normalised — both #1001 and
1001 resolve to the same order. Cross-store lookups return 404,
never 403.