Per MGP_Merchant_Interface_V1.4.7VN.md section 5.3.2: resultCd "00_000"
alone does not mean "payment success" - MGP can send it for a refund
IPN too (status field: "0" = payment, "2" = refund). epayIPN previously
set status="success" purely off resultCd, which would have misclassified
a refund event on a still-pending transaction. Now requires resultCd
"00_000" AND status "0".

Also adds an explicit amount cross-check between the IPN body and the
stored transaction (spec's "Lưu ý đặc biệt 2") as a defense-in-depth,
clearly-diagnosable guard alongside the existing signature verification
(which already implicitly enforces this, but with a generic error).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017KPzWwuTEeX2vXGXvyGn4q