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/.env.example

@@ -47,6 +47,14 @@ WAWP_INSTANCE_ID=your_wawp_instance_id
 DAILY_SUMMARY_ENABLED=true
 SCHEDULED_EMAILS_ENABLED=true
 SCHEDULED_EMAILS_INTERVAL_MS=30000
+# Nightly database backup at 02:00 (requires the `pg_dump` binary on this host).
+# Retention count is set via Admin → Site Settings → Backups (default 14).
+BACKUP_ENABLED=true
+
+# ─── Error monitoring (Sentry) — optional ─────────────────────────────────────
+# Leave unset to disable entirely (a no-op, not an error). Set NEXT_PUBLIC_SENTRY_DSN
+# in frontend/.env too if you want frontend errors captured.
+# SENTRY_DSN=https://xxxxx@oxxxxxx.ingest.sentry.io/xxxxx
 
 # ─── Note ─────────────────────────────────────────────────────────────────────
 # The following are managed via Admin → Site Settings and stored in the database: