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/next.config.ts

@@ -1,4 +1,5 @@
 import type { NextConfig } from "next";
+import { withSentryConfig } from "@sentry/nextjs";
 
 const nextConfig: NextConfig = {
   images: {
@@ -12,4 +13,8 @@ const nextConfig: NextConfig = {
   },
 };
 
-export default nextConfig;
+// A no-op wrap when SENTRY_DSN isn't configured for this deployment — safe in
+// every environment (dev, or a fresh deploy that hasn't set up Sentry yet).
+export default process.env.NEXT_PUBLIC_SENTRY_DSN
+  ? withSentryConfig(nextConfig, { silent: true, disableLogger: true })
+  : nextConfig;