Transfer Anon Order

Transfer a NEW (unpaid) order from an anonymous backend user to the authenticated user.

Called immediately after a guest logs into (or creates) an account via checkout OTP, when Firebase UID linking replaced the anonymous session with a real account and the pre-auth backend user ID differs from the post-auth backend user ID.

Returns 404 if the order is not found on the anonymous user. If the authenticated user already has an active order with items at the same hub, merges the anonymous order into it and returns that existing order. Returns 409 if the order is already paid.