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>

frontend/package.json

@@ -27,6 +27,7 @@
     "@radix-ui/react-tabs": "^1.1.12",
     "@radix-ui/react-toast": "^1.2.14",
     "@radix-ui/react-tooltip": "^1.2.7",
+    "@sentry/nextjs": "^10.71.0",
     "@zxing/browser": "^0.1.5",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",