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/app/events/page.tsx

@@ -1,3 +1,4 @@
+import { Metadata } from "next";
 import { Navbar } from "@/components/layout/Navbar";
 import { Footer } from "@/components/layout/Footer";
 import { EventCard } from "@/components/events/EventCard";
@@ -6,6 +7,11 @@ import { Calendar, CalendarX } from "lucide-react";
 
 export const revalidate = 60;
 
+export const metadata: Metadata = {
+  title: "All Events",
+  description: "Browse and register for upcoming events.",
+};
+
 type Event = {
   id: string;
   title: string;