Fix donation-assignment admin email reading as a real payment

The internal admin notice for applying a donation to a registration
reused buildPaymentAdminNotice as-is, so it read "Payment recorded" /
"Registration payment" — indistinguishable from an actual incoming
payment even though no new money changed hands. Adds a dedicated
buildDonationAssignmentAdminNotice wrapper (mirroring the one already
added for unassignment) so the subject, heading, and Type field all
say "Donation applied" instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/utils/notifications.js

@@ -697,6 +697,29 @@ function buildDonationUnassignedFromRegistrant(payment) {
   return { subject, text, html: emailWrapper(body, { preheader }) };
 }
 
+// Internal admin notice for a donation-assignment — same table layout as
+// buildPaymentAdminNotice (which reads payment.user as "Payer" — for a leg that's the donor,
+// since legs copy userId from the original donation, not the registrant), relabeled so it
+// doesn't read as a fresh incoming payment: no new money changed hands here, an
+// already-recorded donation was just reallocated to a registration.
+function buildDonationAssignmentAdminNotice(payment) {
+  const notice = buildPaymentAdminNotice(payment);
+  const eventTitle = payment.registration?.event?.title || 'Event';
+  const payerName = payment.user?.name || '—';
+  const subject = `Donation applied: ${fmtAmount(payment.amount)} — ${payerName} (${eventTitle})`;
+  return {
+    ...notice,
+    subject,
+    html: notice.html
+      .replace('Payment recorded', 'Donation applied')
+      .replace('Internal notification', 'Internal notification — donation applied to a registration')
+      .replace('>Registration payment<', '>Donation applied<'),
+    text: notice.text
+      .replace('Payment recorded', 'Donation applied')
+      .replace('Type: Registration payment', 'Type: Donation applied'),
+  };
+}
+
 // Internal admin notice for a donation-unassignment — same table layout as
 // buildPaymentAdminNotice (which reads payment.user as "Payer" — for a leg that's the donor,
 // since legs copy userId from the original donation, not the registrant), with copy adjusted
@@ -711,8 +734,11 @@ function buildDonationUnassignmentAdminNotice(payment) {
     subject,
     html: notice.html
       .replace('Payment recorded', 'Donation unassigned')
-      .replace('Internal notification', 'Internal notification — donation removed from registration'),
-    text: notice.text.replace('Payment recorded', 'Donation unassigned'),
+      .replace('Internal notification', 'Internal notification — donation removed from registration')
+      .replace('>Registration payment<', '>Donation unassigned<'),
+    text: notice.text
+      .replace('Payment recorded', 'Donation unassigned')
+      .replace('Type: Registration payment', 'Type: Donation unassigned'),
   };
 }
 
@@ -1057,7 +1083,7 @@ async function sendDonationAssignmentEmails(paymentId) {
     } else {
       sends.push(waTextAny(user, buildWADonationAppliedToRegistrant(payment)));
     }
-    const adminMsg = buildPaymentAdminNotice(payment);
+    const adminMsg = buildDonationAssignmentAdminNotice(payment);
     if (adminMsg.to && adminMsg.to.length) {
       sends.push(sendMail({ to: adminMsg.to.join(','), subject: adminMsg.subject, html: adminMsg.html, text: adminMsg.text }));
     }