Add shared MegaPay signing helper for the admin module

Implements signRequest and signResponse functions that compute
MegaPay/Epay HMAC-SHA256 signatures per MGP_Merchant_Interface
specification sections 5.1 and 5.3. These functions will be consumed
by Tasks 7 and 9 of the admin transaction management feature.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/libs/epaySign.js

+"use strict";
+var sha256 = require("sha256");
+
+function signRequest(timeStamp, merTrxId, merId, amount, encodeKey, payToken) {
+  var base = timeStamp + merTrxId + merId + amount;
+  return payToken
+    ? sha256(base + payToken + encodeKey)
+    : sha256(base + encodeKey);
+}
+
+function signResponse(resultCd, timeStamp, merTrxId, trxId, merId, amount, encodeKey, payToken) {
+  return sha256(
+    resultCd + timeStamp + merTrxId + trxId + merId + amount + (payToken || "") + encodeKey
+  );
+}
+
+module.exports = {
+  signRequest: signRequest,
+  signResponse: signResponse,
+};