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>
     );
 }

frontend/src/app/events/[id]/page.tsx

@@ -1,3 +1,4 @@
+import { Metadata } from "next";
 import { notFound } from "next/navigation";
 import { Navbar } from "@/components/layout/Navbar";
 import { Footer } from "@/components/layout/Footer";
@@ -41,7 +42,7 @@ type Event = {
   location?: string | null;
 };
 
-import { apiFetch, ApiError } from "@/lib/api";
+import { apiFetch, ApiError, resolveToApiOrigin } from "@/lib/api";
 import { ApiImage } from "@/components/shared/ApiImage";
 import { formatDateTimeRange } from "@/lib/date";
 
@@ -105,17 +106,49 @@ function RegisterCta({ event }: { event: Event }) {
   );
 }
 
-export default async function EventDetailPage({ params }: { params: Promise<{ id: string }> }) {
-  const { id } = await params;
-  let event: Event;
+// Next dedupes an identical fetch (same URL + cache options) made during the same
+// request, so calling this again from the page component below is free.
+async function loadEvent(id: string): Promise<Event | null> {
   try {
-    event = await apiFetch<Event>(`/api/events/${id}`, { nextOptions: { next: { revalidate } } });
+    return await apiFetch<Event>(`/api/events/${id}`, { nextOptions: { next: { revalidate } } });
   } catch (e) {
-    // The event endpoint 404s for missing, inactive, or not-yet-live events —
-    // render the standard not-found page instead of crashing.
-    if (e instanceof ApiError && e.status === 404) notFound();
+    if (e instanceof ApiError && e.status === 404) return null;
     throw e;
   }
+}
+
+export async function generateMetadata({ params }: { params: Promise<{ id: string }> }): Promise<Metadata> {
+  const { id } = await params;
+  const event = await loadEvent(id);
+  if (!event) return {};
+
+  const description = event.description
+    ? event.description.slice(0, 200)
+    : `${event.title} — ${formatDateTimeRange(event.startDate, event.endDate)}`;
+  const imageUrl = event.picture ? resolveToApiOrigin(event.picture) : null;
+
+  return {
+    title: event.title,
+    description,
+    openGraph: {
+      title: event.title,
+      description,
+      type: "website",
+      ...(imageUrl ? { images: [{ url: imageUrl }] } : {}),
+    },
+    twitter: {
+      card: "summary_large_image",
+      title: event.title,
+      description,
+      ...(imageUrl ? { images: [imageUrl] } : {}),
+    },
+  };
+}
+
+export default async function EventDetailPage({ params }: { params: Promise<{ id: string }> }) {
+  const { id } = await params;
+  const event = await loadEvent(id);
+  if (!event) notFound();
 
   return (
     <div className="min-h-screen flex flex-col">