Trigger Renewal/Expiration by Shipments Remaining for Print Subscriptions

🔍 Problem Statement

As a publisher managing shipment-based print subscriptions, renewal and expiration are tied to end_date/renewal_date, which don't reflect actual shipments remaining.

💡 User Story

As a publisher, I want auto-renewing (or scheduled-phase) shipment-based subscriptions to activate renewal automatically once shipments remaining hits 0, so subscribers aren't held to an unrelated date.

As a publisher, I want non-auto-renewing shipment-based subscriptions to be marked expired once shipments remaining hits 0, so expiration reflects actual fulfillment status.

🎯 Definition of Done (DoD)

• Given a print subscription is auto-renewing or has a scheduled renewal phase, when shipments remaining reaches 0, then renewal activates regardless of end_date or renewal_date.

• Given a print subscription is non-auto-renewing with no scheduled renewal phase, when shipments remaining reaches 0, then the subscription is marked expired.

• Given an existing print subscription currently tracked by end_date/renewal_date, when this change ships, then it is migrated to shipment-count-based renewal/expiration logic.

• This change will impact API / SDK, specifically shipment-based subscription renewal and expiration logic.

• This solution will include the following limitations: applies only to pure print subscriptions (no print+digital combined pricing plans); end_date and renewal_date are not used as renewal/expiration triggers for shipment-based subscriptions; manual early-activation override is out of scope (superseded by automatic shipment-count trigger).