Add PDF invoices/receipts with manual send, fix early-bird tier edit data loss

- Registration confirmations attach an invoice PDF (itemized breakdown,
  early-bird discount, balance due, Yoco pay-now link/QR) whenever a
  balance is outstanding; payment/donation confirmations attach a
  payment receipt PDF. Sent as an email attachment and, over WhatsApp,
  as the PDF itself with the existing message as its caption.
- Users can also (re)send either document on demand: an "Invoice"
  button on the registration detail popup, and a "Receipt" button next
  to each payment there and on the Payment history page, each opening
  an Email/WhatsApp choice popup, via two new endpoints restricted to
  the registration/payment's own owner.
- Fix: editing an event option's early-bird tiers deleted and
  recreated every tier for that option with brand-new ids, silently
  severing the appliedTierId link on all historical purchases (losing
  early-bird attribution and undercounting stock-limit usage) even for
  tiers the admin didn't touch. Tiers are now upserted by id.
- Update the "My Events" help content and the API docs index for the
  new endpoints.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/controllers/paymentController.js

@@ -1420,6 +1420,20 @@ const getPaymentStats = async (req, res) => {
   }
 }
 
+// @desc    Re-send the receipt PDF for one of the caller's own payments
+// @route   POST /api/payments/:id/send-receipt
+// @access  Private
+const sendReceipt = async (req, res) => {
+  try {
+    const { channel } = req.body || {};
+    const { sendReceiptToUser } = require('../utils/notifications');
+    await sendReceiptToUser(req.params.id, req.user.id, channel);
+    res.json({ message: channel === 'whatsapp' ? 'Receipt sent to WhatsApp.' : 'Receipt emailed.' });
+  } catch (error) {
+    res.status(error.statusCode || 400).json({ message: error.message });
+  }
+};
+
 module.exports = {
   createPayment,
   getPayments,
@@ -1433,5 +1447,6 @@ module.exports = {
   createRegistrationCheckoutInternal,
   sendPaymentLink,
   createRefund,
-  getPaymentStats
+  getPaymentStats,
+  sendReceipt,
 };