Add a 'Products' field to the Coupon edit page alongside the 'Subscription Plans' field.

🔍 Problem Statement

The Coupon edit page only allows associating coupons with specific subscription plans — not with products. Admins who want a coupon to apply to all plans under a product must individually select each plan, which is tedious and error-prone when plans are added or removed.

💡 User Story

As a Pelcro admin, I want to associate a coupon with a product (not just individual plans), so that the coupon automatically applies to all current and future plans under that product.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given the coupon edit page, when an admin selects a product, then the coupon applies to all plans under that product — including any plans added to the product in the future.