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/index.js

@@ -529,6 +529,15 @@ app.get('/docs', async (req, res) => {
         pathParams:{ ':id':'Registration UUID' },
         request:{ body:{ answers:[{ fieldId:'field-uuid-...', value:'Yes' }, { fieldId:'field-uuid-2', value:'Vegetarian' }] }},
         responses:[{ status:201, desc:'Submitted', body:{ id:'response-uuid-...', createdAt:'2025-06-01T10:05:00.000Z' }}]},
+      { method:'POST', path:'/api/registrations/:id/send-invoice', auth:'user+', desc:'(Re)generate the invoice PDF for one of the caller\'s own registrations and send it on the requested channel — email or WhatsApp. Works regardless of balance (shows "Paid in full" when nothing is owed); fails only if there is no valid email/phone on file for the chosen channel.',
+        notes:'The PDF\'s invoiceNo is derived, not sequential: INV-<year of registration.createdAt>-<last 6 hex chars of the registration id, uppercased>. When there is an outstanding balance, generates a fresh Yoco checkout link for the invoice\'s QR code/pay-now link on each call.',
+        pathParams:{ ':id':'Registration UUID' },
+        request:{ body:{ channel:'email' }},
+        responses:[
+          { status:200, desc:'Sent', body:{ message:'Invoice emailed.' }},
+          { status:400, desc:'No valid contact for channel', body:{ message:'No valid email address on file. Add one in your profile.' }},
+          { status:403, desc:'Not the owner', body:{ message:'Not authorized to access this registration' }},
+        ]},
     ]},
 
     { title: 'Payments', base: '/api/payments', endpoints: [
@@ -546,6 +555,14 @@ app.get('/docs', async (req, res) => {
       { method:'GET', path:'/api/payments/mypayments', auth:'user+', desc:'Get own payment history (paginated, excludes donations). Returned method is normalized to cash|card|eft|voucher|other — apple_pay/google_pay report as "card", any other gateway-reported value reports as "other"',
         queryParams:{ page:'Page (default 1)', limit:'Per page (default 25, max 25)', startDate:'ISO date, filters createdAt >=', endDate:'ISO date, filters createdAt <=', method:'Filter by normalized method: cash|card|eft|voucher|other', kind:'payment|refund — filters by amount sign' },
         responses:[{ status:200, desc:'Success', body:{ data:[{ id:'pay-uuid-...', amount:450, method:'card', status:'succeeded', createdAt:'2025-06-01T11:00:00.000Z' }], total:1, page:1, limit:25, pages:1 }}]},
+      { method:'POST', path:'/api/payments/:id/send-receipt', auth:'user+', desc:'Regenerate the receipt PDF for one of the caller\'s own payments and send it on the requested channel — email or WhatsApp. Fails if there is no valid email/phone on file for the chosen channel.',
+        notes:'The PDF\'s receiptNo is derived, not sequential: RCPT-<year of payment.createdAt>-<last 6 hex chars of the payment id, uppercased>.',
+        pathParams:{ ':id':'Payment UUID' },
+        request:{ body:{ channel:'whatsapp' }},
+        responses:[
+          { status:200, desc:'Sent', body:{ message:'Receipt sent to WhatsApp.' }},
+          { status:403, desc:'Not the owner', body:{ message:'Not authorized to access this payment' }},
+        ]},
       { method:'GET', path:'/api/payments', auth:'supervisor+', desc:'List all payments',
         queryParams:{ page:'Page (default 1)', limit:'Per page (default 20)', eventId:'Filter by event', userId:'Filter by user', method:'Filter by method (cash|card|eft|donation)', startDate:'ISO date', endDate:'ISO date' },
         responses:[{ status:200, desc:'Success', body:{ data:[{ id:'pay-uuid-...', amount:450, method:'card', user:{ name:'Jane Doe' }, registration:{ event:{ title:'Camp 2025' }}}], total:1 }}]},