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/src/controllers/registrationController.js

@@ -5,6 +5,8 @@ const { emailTickets } = require('./ticketController');
 const { hashPassword } = require('../config/auth');
 const { resolveOptionPrice, resolveVariantTierPrice, computeRegistrationTotalDue, refreshPricingForRegistration, attachComputedTotals, attachComputedTotalsToList } = require('../utils/pricing');
 const { assertEventOpen } = require('../utils/cashupUtils');
+const { logAdminAction } = require('../utils/adminAudit');
+const { getClientIp } = require('../utils/requestUtils');
 
 /**
  * Check overall stock availability for an EventOption.
@@ -698,6 +700,20 @@ const cancelRegistration = async (req, res) => {
       }
     });
 
+    // Only log when staff cancels on someone else's behalf — a routine self-service
+    // cancellation isn't an admin action worth cluttering the audit trail with.
+    if (registration.userId !== req.user.id) {
+      logAdminAction({
+        actorId: req.user.id,
+        actorRole: req.user.role,
+        action: 'registration_cancelled',
+        targetType: 'Registration',
+        targetId: req.params.id,
+        metadata: { registrationOwnerId: registration.userId },
+        ip: getClientIp(req),
+      });
+    }
+
     res.json({ message: 'Registration cancelled', registration: updatedRegistration });
   } catch (error) {
     res.status(400).json({ message: error.message });
@@ -1143,6 +1159,16 @@ const createManualRegistration = async (req, res) => {
       }
     })();
 
+    logAdminAction({
+      actorId: req.user.id,
+      actorRole: req.user.role,
+      action: 'registration_created_manual',
+      targetType: 'Registration',
+      targetId: registrationId,
+      metadata: { eventId: registration.eventId, forUserId: userId },
+      ip: getClientIp(req),
+    });
+
     return res.status(201).json(attachComputedTotals(registration));
   } catch (error) {
     console.error(error);