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/paymentController.js

@@ -2,6 +2,9 @@ const prisma = require('../config/db');
 const { v4: uuidv4 } = require('uuid');
 const { generateTicketsForRegistration } = require('../utils/ticketUtils');
 const { computeRegistrationTotalDue, refreshPricingForRegistration } = require('../utils/pricing');
+const { computeDonationRemaining } = require('../utils/donationUtils');
+const { logAdminAction } = require('../utils/adminAudit');
+const { getClientIp } = require('../utils/requestUtils');
 const axios = require('axios');
 const { emailTickets } = require('./ticketController');
 const { safeErrorMessage } = require('../utils/errorUtils');
@@ -611,17 +614,12 @@ const assignDonationToRegistration = async (req, res) => {
       throw new Error('Only donations can be assigned to registrations');
     }
 
-    // Donations are never mutated once created — their remaining balance is the original
-    // amount minus every leg (a Payment row with isDonation:false and originalPaymentId
-    // pointing back at this donation) already allocated from it. A refund of the donation
-    // itself also creates such a leg, with a negative amount — Math.abs() so a refund reduces
-    // the remaining balance (money that's left the building) instead of increasing it (which a
-    // raw signed sum would do, since subtracting a negative adds).
+    // See computeDonationRemaining's doc comment for why refund legs (negative amount) reduce
+    // rather than inflate the remaining balance.
     const existingLegs = await prisma.payment.findMany({
       where: { originalPaymentId: payment.id, isDonation: false }
     });
-    const alreadyUsed = existingLegs.reduce((sum, leg) => sum + Math.abs(leg.amount), 0);
-    const remainingDonation = payment.amount - alreadyUsed;
+    const remainingDonation = computeDonationRemaining(payment.amount, existingLegs);
 
     if (remainingDonation <= 0.000001) {
       res.status(400);
@@ -766,6 +764,16 @@ const assignDonationToRegistration = async (req, res) => {
       donationRemaining: remainingDonation - allocateAmount
     };
 
+    logAdminAction({
+      actorId: req.user.id,
+      actorRole: req.user.role,
+      action: 'donation_assigned',
+      targetType: 'Registration',
+      targetId: registrationId,
+      metadata: { paymentId, legId: leg.id, allocateAmount },
+      ip: getClientIp(req),
+    });
+
     res.status(200).json(result);
   } catch (error) {
     res.status(res.statusCode === 200 ? 400 : res.statusCode).json({ message: safeErrorMessage(error) });
@@ -885,6 +893,16 @@ const unassignDonationFromRegistration = async (req, res) => {
       catch (e) { console.error('Failed to send emails after unassigning donation:', e); }
     })();
 
+    logAdminAction({
+      actorId: req.user.id,
+      actorRole: req.user.role,
+      action: 'donation_unassigned',
+      targetType: 'Registration',
+      targetId: leg.registrationId,
+      metadata: { legId: leg.id, donationId: donation.id, amount: leg.amount },
+      ip: getClientIp(req),
+    });
+
     return res.status(200).json({
       message: 'Donation unassigned',
       updatedRegistration: finalRegistration,
@@ -1357,6 +1375,16 @@ const createRefund = async (req, res) => {
     const { sendRefundEmail } = require('../utils/notifications');
     sendRefundEmail(negativePayment.id).catch(e => console.error('Failed to send refund email:', e));
 
+    logAdminAction({
+      actorId: req.user.id,
+      actorRole: req.user.role,
+      action: 'refund_created',
+      targetType: 'Payment',
+      targetId: negativePayment.id,
+      metadata: { amount: amt, method: method || 'refund', reason: reason || null, registrationId: linkRegistrationId },
+      ip: getClientIp(req),
+    });
+
     return res.status(201).json(negativePayment);
   } catch (error) {
     res.status(res.statusCode === 200 ? 400 : res.statusCode).json({ message: safeErrorMessage(error) });