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/requestUtils.js

@@ -0,0 +1,9 @@
+// Resolve a client IP from the request (works behind proxies) — mirrors the equivalent
+// local helper in userController.js, shared here for the admin-audit call sites.
+function getClientIp(req) {
+  const forwarded = req.headers['x-forwarded-for'];
+  if (forwarded) return forwarded.split(',')[0].trim();
+  return req.socket?.remoteAddress || 'unknown';
+}
+
+module.exports = { getClientIp };