Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Checkpoint. Pick the mode and price the job. Write plan(items, user_waiting) that chooses the mode (realtime if waiting, else batch),
prices it ($0.01/item realtime, $0.002/item batch), and returns:
batch for 5000 items: $10.0
Format "{mode} for {items} items: ${cost}".