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/routes/registrationRoutes.js

@@ -12,6 +12,7 @@ const {
   updateRegistrationOptions,
   submitFormResponses, replaceFormResponses,
   getFormDraft, saveFormDraft,
+  sendInvoice,
 } = require('../controllers/registrationController');
 const { protect, supervisor, staff, optionalAuth } = require('../middleware/authMiddleware');
 
@@ -20,6 +21,7 @@ router.post('/', optionalAuth, createRegistration);
 router.get('/myregistrations', protect, getUserRegistrations);
 router.put('/:id/options', protect, updateRegistrationOptions);
 router.delete('/:id', protect, cancelRegistration);
+router.post('/:id/send-invoice', protect, sendInvoice);
 
 // Registration detail + forms — optionalAuth so guests can access with just the registrationId
 router.get('/:id', optionalAuth, getRegistrationById);