Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Checkpoint. Build a serving handler with HTTP-style status. Write
serve(request) that returns "400 missing amount" if amount is
absent; otherwise scores it (amount * 0.01) and returns:
200 fraud (0.95)
Format "200 {label} ({score})" where label is fraud if score > 0.5
else legit, and score is rounded to 2 decimals.