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>

SETTINGS.md

@@ -74,6 +74,12 @@ or Branding tab has been saved even once — the wizard always writes
 |---|---|---|
 | `WAWP_ACCESS_TOKEN` / `WAWP_INSTANCE_ID` | `WAWP_ACCESS_TOKEN`/`WAWP_INSTANCE_ID` env vars, else *(blank)* | No hardcoded default — WhatsApp sending is simply unavailable until both are configured (Admin → Site Settings → WhatsApp). |
 
+## Backups
+
+| Setting | Default when unset | Notes |
+|---|---|---|
+| `backup_retain_count` | `14` | How many nightly backups to keep on disk (oldest deleted beyond this count). Editable at Admin → Site Settings → Backups. See `BACKUP_ENABLED` (env var) to turn the nightly job off entirely. |
+
 ## Setup state
 
 | Setting | Default when unset | Notes |