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/src/instrumentation-client.ts

@@ -0,0 +1,17 @@
+// A no-op when NEXT_PUBLIC_SENTRY_DSN isn't set, so this is safe in every environment
+// (dev, or a fresh deploy that hasn't configured Sentry yet). The dynamic import (rather
+// than a top-level `import * as Sentry`) keeps the Sentry client SDK out of every visitor's
+// bundle entirely when it's unconfigured — Next inlines NEXT_PUBLIC_* at build time, so an
+// unset DSN lets the bundler dead-code-eliminate this whole block, import included.
+if (process.env.NEXT_PUBLIC_SENTRY_DSN) {
+  const dsn = process.env.NEXT_PUBLIC_SENTRY_DSN;
+  import("@sentry/nextjs").then((Sentry) => {
+    Sentry.init({
+      dsn,
+      environment: process.env.NODE_ENV || "development",
+      // Small single-VM deployment, not high-traffic — start conservative and raise
+      // this once real usage is visible in Sentry, rather than sampling every request.
+      tracesSampleRate: 0.1,
+    });
+  });
+}