Add applied coupon to subscriptions list export

🔍 Problem Statement

As an operations or reporting user, I can see the applied coupon code in the subscriptions UI list, but the same information is missing from the subscriptions export, which results in incomplete reporting and manual reconciliation work.

💡 User Story

As an operations or finance user, I want the applied coupon code to be included in the subscriptions export, so that I can analyze discounts, promotions, and subscription data accurately without relying on the UI manually.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a subscription has an applied coupon or discount code, when the subscriptions export is generated, then the export includes the applied coupon code value in a dedicated column.

✔️ Given a subscription does not have an applied coupon code, when the export is generated, then the coupon code column remains empty/null for that record.

✔️ Given the coupon code is visible in the subscriptions UI list, when exporting the same dataset, then the export data matches the UI data for coupon-related fields.

1. new column added to end of export

✔️ This change will impact:

1. UI / Export Service / Reporting

2. Specifically:

3. Subscriptions export generation logic

4. Export CSV/XLSX column mappings

5. Subscription data serializer/API used for exports

✔️ The export will include:

1. A new column such as applied_coupon_code

2. Coupon value exactly as stored/displayed in the subscriptions UI