The live Stripe webhook endpoint has NO pinned api_version, so it follows the
account default (currently 2024-12-18.acacia), which delivers the subscription
link as the top-level invoice.subscription. The code only read the new
2026-03-25.dahlia shape (invoice.parent.subscription_details.subscription), so
recurring renewal/payment-failed events would have returned a null subscription
id and silently failed to fulfill once the events were enabled.
invoiceSubscriptionId() now reads the modern shape first, then falls back to the
legacy top-level field. All other invoice fields used by the handlers
(amount_due, attempt_count, hosted_invoice_url, id) are stable across both
versions. +5 tests (legacy string/object, modern-preferred-over-legacy).
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