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/dashboard/admin/page.tsx

@@ -8,7 +8,7 @@ import { useStableState } from "@/hooks/useStableState";
 import { useVisiblePolling } from "@/hooks/useVisiblePolling";
 import {
   Calendar, Banknote, Gift, Users, Ticket, QrCode, ClipboardList,
-  UserPlus, FileText, MessageCircle, BarChart2, Mail, Wallet, DoorOpen,
+  UserPlus, FileText, MessageCircle, BarChart2, Mail, Wallet, DoorOpen, History,
 } from "lucide-react";
 import { StatCard, StatCardRow } from "@/components/shared/StatCard";
 import { QuickActionTile, QuickActionGrid } from "@/components/shared/QuickActionTile";
@@ -33,6 +33,7 @@ const QUICK_ACTIONS = [
   { href: "/dashboard/supervisor/email-attendees", label: "Email attendees", description: "Send a message to attendees of an event", icon: Mail },
   { href: "/dashboard/supervisor/whatsapp-attendees", label: "WhatsApp attendees", description: "Send a WhatsApp message to event attendees", icon: MessageCircle },
   { href: "/dashboard/admin/cashup", label: "Post-event Cashup", description: "Set costs, reconcile takings, and close out events", icon: Wallet },
+  { href: "/dashboard/admin/audit-log", label: "Audit log", description: "Review refunds, manual registrations, event and settings changes", icon: History },
 ] as const;
 
 type OverviewMetric = { thisMonth: number; lastMonth: number; pctChange: number | null };