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/layout.tsx

@@ -10,6 +10,7 @@ import { SetupGuard } from "@/components/shared/SetupGuard";
 import HelpFab from "@/components/shared/HelpFab";
 import { API_BASE, resolveToApiOrigin } from "@/lib/api";
 import { buildThemeCssVars } from "@/lib/colorScale";
+import { appUrl } from "@/lib/siteConfig";
 
 const geistSans = Geist({
   variable: "--font-geist-sans",
@@ -44,14 +45,30 @@ async function getServerSettings(): Promise<SiteSettings> {
 export async function generateMetadata(): Promise<Metadata> {
   const settings = await getServerSettings();
   const faviconUrl = settings.favicon_url ? resolveToApiOrigin(settings.favicon_url) : null;
+  const logoUrl = settings.logo_url ? resolveToApiOrigin(settings.logo_url) : null;
   const displayName = settings.org_name || appName;
+  const description = settings.org_tagline || `Manage and register for events with ${displayName}`;
 
   return {
-    title: displayName,
-    description: `Manage and register for events with ${displayName}`,
+    metadataBase: new URL(appUrl),
+    title: { default: displayName, template: `%s | ${displayName}` },
+    description,
     icons: {
       icon: faviconUrl || "/favicon.ico",
     },
+    openGraph: {
+      type: "website",
+      siteName: displayName,
+      title: { default: displayName, template: `%s | ${displayName}` },
+      description,
+      ...(logoUrl ? { images: [{ url: logoUrl }] } : {}),
+    },
+    twitter: {
+      card: "summary_large_image",
+      title: displayName,
+      description,
+      ...(logoUrl ? { images: [logoUrl] } : {}),
+    },
   };
 }