Sync shipments remaining to Mailchimp

🔍 Problem Statement

The 'shipments remaining' count on a subscription is not synced to Mailchimp. Publishers using Mailchimp for fulfilment communications cannot segment or trigger emails based on how many shipments a subscriber has left.

💡 User Story

As a publisher using Mailchimp, I want shipments remaining to sync to Mailchimp as a merge field, so that I can segment subscribers and trigger emails based on remaining fulfilment.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a subscription with shipments, when the shipments remaining count changes, then the updated value is synced to the corresponding Mailchimp subscriber's merge field.