Search Payments by Last 4 Digits of Credit Card

🔍 Problem Statement
As a billing admin, I experience being unable to search payments by the last four digits of a credit card, which makes it difficult to match a payment when the cardholder's identity is unknown.

💡 User Story
As a billing admin, I want to search the Payments screen by the last 4 digits of a credit card, so that I can quickly locate a payment when a cardholder contacts us without knowing their account details.

🎯 Definition of Done
- Given the Payments search, when filtering by last 4 card digits, then only payments made with that card are returned.
- This change impacts the UI (Payments search/filter) and API (payment search endpoint — add last4 filter param).
- Limitations: returns all matching cards across customers; no full PAN data is stored or exposed.