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>

backend/prisma/migrations/20260827114148_add_admin_audit_log/migration.sql

@@ -0,0 +1,29 @@
+-- CreateEnum
+CREATE TYPE "AdminAuditAction" AS ENUM ('refund_created', 'donation_assigned', 'donation_unassigned', 'registration_created_manual', 'registration_cancelled', 'event_created', 'event_updated', 'event_deleted', 'settings_updated');
+
+-- CreateTable
+CREATE TABLE "AdminAuditLog" (
+    "id" TEXT NOT NULL,
+    "actorId" TEXT,
+    "actorRole" TEXT NOT NULL,
+    "action" "AdminAuditAction" NOT NULL,
+    "targetType" TEXT NOT NULL,
+    "targetId" TEXT,
+    "metadata" JSONB,
+    "ip" TEXT,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "AdminAuditLog_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "AdminAuditLog_actorId_createdAt_idx" ON "AdminAuditLog"("actorId", "createdAt");
+
+-- CreateIndex
+CREATE INDEX "AdminAuditLog_action_createdAt_idx" ON "AdminAuditLog"("action", "createdAt");
+
+-- CreateIndex
+CREATE INDEX "AdminAuditLog_targetType_targetId_idx" ON "AdminAuditLog"("targetType", "targetId");
+
+-- AddForeignKey
+ALTER TABLE "AdminAuditLog" ADD CONSTRAINT "AdminAuditLog_actorId_fkey" FOREIGN KEY ("actorId") REFERENCES "User"("id") ON DELETE SET NULL ON UPDATE CASCADE;