helcimFetch called response.json() unconditionally, which threw
"Unexpected end of JSON input" on Helcim's 204 No Content responses
(e.g. DELETE /subscriptions/:id). The error was silently swallowed by
the best-effort cancel path in cancel-subscription, masking cases where
the Helcim-side cancel actually succeeded.
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.
