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/routes/eventRoutes.js

@@ -6,6 +6,7 @@ const {
   getAllEvents,
   getEventsAll,
   getEventById,
+  getEventIcs,
   updateEvent,
   getEventNotifyRecipients,
   updateEventNotifyRecipients,
@@ -44,6 +45,7 @@ router.post('/attachments/sync', protect, admin, attachmentsSync);
 // optionalAuth populates req.user when a valid token is present so staff/supervisor/admin
 // can still load inactive events (e.g. for cashup or editing) without being 404'd.
 router.get('/:id', optionalAuth, getEventById);
+router.get('/:id/ics', optionalAuth, getEventIcs);
 router.get('/by-alias/:redirectUrl', getEventByAlias);
 
 // Create/Update/Delete event