In the Case 1 (free→paid) branch of update-contribution, after the
subscription is created and the member row is updated, fetch the
newest paid Helcim transaction and upsert a Payment with
paymentType=cadence and sendConfirmation=true.
Paid→paid (Case 3) is intentionally NOT wired — no new transaction
occurs at amount change; the next recurring charge is captured by the
reconciliation script.
Persist nextBillingDate on subscription create/update; unset on
cancel or downgrade to free. Account page displays the cached
date and lazily refreshes from Helcim when the cached value is
within 24h of now (or missing).
