Display One-Time Coupons at Subscription Level in Admin

🔍 Problem Statement

When a one-time coupon is applied to a subscription, it is not visible at the subscription level in the admin dashboard. Admins cannot see which coupon was used or its value without digging into invoice details, making it hard to audit promotional activity.

💡 User Story

As a Pelcro admin, I want one-time coupons to be displayed on the subscription detail page, so that I can quickly see which promotional codes were applied without navigating to individual invoices.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a subscription with a one-time coupon applied, when an admin views the subscription detail page, then the coupon name, code, discount value, and application date are visible.