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/package.json

@@ -6,7 +6,7 @@
   "scripts": {
     "start": "node src/index.js",
     "dev": "nodemon src/index.js",
-    "test": "echo \"Error: no test specified\" && exit 1",
+    "test": "jest",
     "postinstall": "prisma generate",
     "prisma:generate": "prisma generate",
     "prisma:deploy": "prisma migrate deploy && prisma generate",
@@ -17,12 +17,14 @@
   },
   "dependencies": {
     "@prisma/client": "^5.4.2",
+    "@sentry/node": "^10.71.0",
     "axios": "^1.11.0",
     "bcryptjs": "^2.4.3",
     "cors": "^2.8.5",
     "dotenv": "^16.3.1",
     "exceljs": "^4.4.0",
     "express": "^4.18.2",
+    "ics": "^3.12.0",
     "jsonwebtoken": "^9.0.2",
     "multer": "^2.0.2",
     "node-fetch": "^2.7.0",
@@ -33,6 +35,7 @@
     "uuid": "^9.0.1"
   },
   "devDependencies": {
+    "jest": "^30.4.2",
     "nodemon": "^3.0.1",
     "prisma": "^5.4.2"
   }