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/icsUtils.js

@@ -0,0 +1,36 @@
+const { createEvent } = require('ics');
+
+// Event.startDate/endDate are stored as true UTC instants (the admin event form's
+// datetime-local input is parsed in the browser's local time before being sent as an
+// ISO string), so serializing them as UTC here requires no timezone math and lets every
+// viewer's calendar app localize correctly to *their own* timezone.
+function toUtcArray(date) {
+  const d = new Date(date);
+  return [d.getUTCFullYear(), d.getUTCMonth() + 1, d.getUTCDate(), d.getUTCHours(), d.getUTCMinutes()];
+}
+
+/**
+ * Build an RFC 5545 .ics file (as a string) for a single event.
+ *
+ * @param {object} event - Prisma Event row: title, description?, startDate, endDate, location?
+ * @param {string} eventUrl - absolute URL to the event's public page
+ * @returns {string}
+ */
+function buildEventIcs(event, eventUrl) {
+  const { error, value } = createEvent({
+    title: event.title,
+    start: toUtcArray(event.startDate),
+    end: toUtcArray(event.endDate),
+    startInputType: 'utc',
+    endInputType: 'utc',
+    startOutputType: 'utc',
+    endOutputType: 'utc',
+    location: event.location || undefined,
+    description: event.description || undefined,
+    url: eventUrl,
+  });
+  if (error) throw error;
+  return value;
+}
+
+module.exports = { buildEventIcs };