Replace tier-based plan lookup with cadence-keyed lookup, compute
recurringAmount via getTierAmount, persist billingCadence on member.
Delete both manual-fallback blocks; Helcim failure now surfaces as 500.
Moves updateHelcimSubscription to the live-verified wrapped shape
(PATCH /subscriptions { subscriptions: [{ id, ...payload }] }), adds a prior-
status check so sendWelcomeEmail only fires on pending_payment to active
transitions, short-circuits get-or-create-customer when a valid
helcimCustomerId is already on file, and replaces member.save() Slack-status
writes with findByIdAndUpdate({ runValidators: false }) to avoid save-time
validator pitfalls.
