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/[id]/ClientActions.tsx

@@ -1,7 +1,8 @@
 "use client";
 
-import { Share2, QrCode } from "lucide-react";
+import { Share2, QrCode, CalendarPlus } from "lucide-react";
 import QRCode from "qrcode";
+import { API_BASE } from "@/lib/api";
 
 export default function ClientActions({ event }: { event: any }) {
     return (
@@ -43,6 +44,14 @@ export default function ClientActions({ event }: { event: any }) {
                 <QrCode className="w-4 h-4" />
                 Save QR
             </button>
+            <a
+                href={`${API_BASE}/api/events/${event.id}/ics`}
+                download
+                className="flex items-center gap-2 px-4 py-2 bg-gray-100 rounded-lg hover:bg-gray-200 transition-colors text-sm font-medium"
+            >
+                <CalendarPlus className="w-4 h-4" />
+                Add to calendar
+            </a>
         </div>
     );
 }