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>

frontend/src/app/dashboard/supervisor/event-options/page.tsx

@@ -21,7 +21,7 @@ function toLocalDateTimeInputValue(input: string | number | Date | null | undefi
   return `${y}-${m}-${day}T${hh}:${mm}`;
 }
 
-function EarlyBirdTiersEditor({ option, onSave }: { option: any; onSave: (tiers: { deadline: string; price: number; order?: number }[]) => void }) {
+function EarlyBirdTiersEditor({ option, onSave }: { option: any; onSave: (tiers: { id?: string; deadline: string; price: number; order?: number }[]) => void }) {
   const [rows, setRows] = React.useState<{ id?: string; deadline: string; price: string; order?: number }[]>([]);
   const [open, setOpen] = React.useState(false);
   const [saving, setSaving] = React.useState(false);
@@ -54,7 +54,7 @@ function EarlyBirdTiersEditor({ option, onSave }: { option: any; onSave: (tiers:
     try {
       const tiers = rows
         .filter((r) => !!r.deadline && String(r.price).trim() !== '')
-        .map((r, i) => ({ deadline: new Date(r.deadline).toISOString(), price: parseFloat(r.price), order: typeof r.order === 'number' ? r.order : i }))
+        .map((r, i) => ({ id: r.id, deadline: new Date(r.deadline).toISOString(), price: parseFloat(r.price), order: typeof r.order === 'number' ? r.order : i }))
         .filter((t) => t.price >= 0 && !isNaN(new Date(t.deadline).getTime()));
       onSave(tiers);
     } finally {