Add calendar export, SEO, error monitoring, backups, audit trail, and a starter test suite

Six site improvements picked from a "what could be better" review, plus a Jest
test suite covering the two areas with the trickiest money-handling history
in this project (early-bird pricing tranches, donation-leg accounting):

- "Add to calendar" .ics download on event pages and in confirmation emails
- sitemap.xml, robots.txt, and Open Graph/Twitter metadata for public pages
- Sentry error monitoring (backend + frontend), a no-op until SENTRY_DSN is set
- Nightly local pg_dump backups with a Site Settings tab to browse/trigger/download
- Admin audit trail for refunds, donations, manual registrations, event and
  settings changes, and staff-initiated cancellations
- Jest tests reproducing and guarding against the 1.8.0 tranche-pricing bug
  and the 1.4.2 donation-balance-inflation bug

Wallet passes (Google/Apple) were scoped out of this round — Apple Wallet
needs a paid Apple Developer account the project doesn't have yet, and the
user preferred shipping both together later rather than Google alone now.

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

backend/src/utils/notifications.js

@@ -1,6 +1,6 @@
 const fs = require('fs');
 const prisma = require('../config/db');
-const { sendMail, emailWrapper, ctaButton, fallbackLink, divider, callout, paymentOption } = require('./email');
+const { sendMail, emailWrapper, ctaButton, fallbackLink, secondaryLink, divider, callout, paymentOption } = require('./email');
 const { computeRegistrationTotalDue, computeOptionLineTotal } = require('./pricing');
 
 // ─── Formatting helpers ───────────────────────────────────────────────────────
@@ -22,6 +22,20 @@ function fmtDateShort(d) {
 
 const { getSettingSync } = require('./settingsCache');
 
+// The .ics calendar-download link lives on the backend (not the frontend site), same
+// as the ticket-PDF URLs sent to WhatsApp — see whatsapp.js's BACKEND_URL usage.
+function getBackendUrl() {
+  return (process.env.BACKEND_URL || '').replace(/\/$/, '');
+}
+
+// Only rendered when BACKEND_URL is actually configured — the .ics endpoint lives on
+// the backend, and there's no reliable way to derive that URL otherwise.
+function calendarLinkRow(eventId) {
+  const backendUrl = getBackendUrl();
+  if (!backendUrl || !eventId) return '';
+  return secondaryLink('Add to calendar', `${backendUrl}/api/events/${eventId}/ics`);
+}
+
 function getOrg() {
   return {
     name:  getSettingSync('org_name', process.env.ORG_NAME || 'Cross Code'),
@@ -269,7 +283,8 @@ function buildRegistrationConfirmation(reg, { isNew = true } = {}) {
     ${financialSummary(totalDue, totalPaid, balance)}
 
     ${paymentSection({ balance, yocoLink: null, source: 'user', siteUrl: org.url, formRequired: false, isUserActive })}
-    ${accountCta(isUserActive, org.url)}`;
+    ${accountCta(isUserActive, org.url)}
+    ${calendarLinkRow(reg.eventId)}`;
 
   const itemsText = (reg.registrationOptions || []).map(ro => `  • ${ro.eventOption?.name || 'Option'} ×${ro.quantity} — ${fmtAmount(computeOptionLineTotal(ro, null, new Date()))}`).join('\n');
   const text = `${heading}\n\nHi ${reg.user?.name || 'there'},\n\n${isNew ? `You are registered for ${eventTitle}` : `Your registration for ${eventTitle} has been updated`}${eventDate ? ' on ' + eventDate : ''}.\n\nYour selections:\n${itemsText || '  —'}\n\nTotal due: ${fmtAmount(totalDue)}\nAmount paid: ${fmtAmount(totalPaid)}\nBalance: ${fmtAmount(balance)}\n\n${balance > 0 ? `Payment options:\n  1. On our website: ${org.url}\n  2. At the door (cash or card)\n\nYour tickets will be sent once payment is confirmed.` : 'No payment required — your tickets have been sent separately.'}\n\n${org.name} — ${org.email}\n${org.url}`;
@@ -309,7 +324,8 @@ function buildAdminInitiatedRegistrationConfirmation(reg, { yocoLink = null, for
     ${financialSummary(totalDue, totalPaid, balance)}
 
     ${paymentSection({ balance, yocoLink, source: 'admin', siteUrl: org.url, formRequired, isUserActive })}
-    ${accountCta(isUserActive, org.url)}`;
+    ${accountCta(isUserActive, org.url)}
+    ${calendarLinkRow(reg.eventId)}`;
 
   const itemsText = (reg.registrationOptions || []).map(ro => `  • ${ro.eventOption?.name || 'Option'} ×${ro.quantity} — ${fmtAmount(computeOptionLineTotal(ro, null, new Date()))}`).join('\n');
   const payText = balance > 0