Multi-subscription merge tags for Mailchimp integration

🔍 Problem Statement

As an email marketer at Les Débrouillardes, I can't segment or personalize campaigns per subscription when a customer holds multiple subscriptions, since merge tags only hold the most recent one.

💡 User Story

As an email marketer, I want per-subscription merge tags for customers with multiple active subscriptions, so that I can build campaigns targeting specific products (DEB, EXP, CURIUM).

🎯 Definition of Done (DoD)

✔ Given a customer with 2+ active subscriptions, when Mailchimp sync runs, then each subscription's data populates its own indexed merge tag set (e.g. PRODNAME_1, PLANNAME_1, SUBSTATUS_1, PRODNAME_2, PLANNAME_2, SUBSTATUS_2).

✔ This change will impact Integration Orchestrator and API, specifically the Mailchimp sync connector and merge tag mapping logic.

✔ This solution will include the following limitations: indexed tags support up to [TBD - clarify max subscriptions to support, e.g. 5] subscriptions per customer; existing single-subscription merge tags remain unchanged for backward compatibility.