Function derivePayZenSubscriptionStatus

The V4/Subscription object has NO status field — the unified status is derived from the facts it does carry, first match wins:

1. cancelDate set → "canceled" (Subscription/Cancel stamps it);
2. totalPaymentsNumber > 0 and pastPaymentsNumber has reached it → "completed" (a finite rrule — COUNT/UNTIL — ran all installments; open-ended subscriptions report totalPaymentsNumber 0 and never complete);
3. effectDate in the future → "pending" (created, not yet billing);
4. otherwise → "active".