Add a Segment filter for current invoice amount

🔍 Problem Statement

As a publisher, I can't segment subscriptions by the price they currently pay: the Price filter matches any past invoice, and the Plan amount filter checks the plan price only, ignoring coupons, so renewal segments overlap or target the wrong price.

💡 User Story

As a publisher, I want a new segment filter that checks only the amount of the subscription's current invoice, so that I can target renewal offers by the price each subscriber currently pays.

🎯 Definition of Done (DoD)

A feature is done when:

✔ When the user adds the current invoice amount filter in the segment, then only subscriptions whose current invoice matches are included, regardless of past invoices.

✔ The filter matches the invoice amount after coupons and discounts.

✔ The filter supports an exact amount or a range, like the Price filter.