Add merchant payment status filter

[awyoo]

Summary

• add status_filter: Option<PaymentStatus> to get_merchant_payments_paginated
• filter stored merchant payment IDs by PaymentCharge.status before applying pagination
• preserve unfiltered behavior with None
• add coverage for unfiltered, pending, confirmed, pagination-after-filtering, and empty-result cases
• update local invoke docs with the new optional filter argument

/claim #280

Payout address: 0xe80506A1431B3B4aAca8B260838481deBbdF75Ab

Verification

• git diff --check
• Not run: cargo test -p fluxapay test_get_merchant_payments_paginated -- --nocapture because this local environment does not have cargo/rustc installed.