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/eventController.js

@@ -691,26 +691,49 @@ const updateEventOption = async (req, res) => {
       }
     });
 
-    // If earlyBirdTiers provided, replace all tiers for this option (including per-variant ones)
+    // If earlyBirdTiers provided, upsert tiers for this option (including per-variant ones) by
+    // id — mirrors the variants upsert just below. Editing tiers used to delete-and-recreate
+    // every tier for the option on every save, which silently reassigned each one a brand-new
+    // id; RegistrationOption/RegistrationOptionTranche.appliedTierId (onDelete: SetNull) then
+    // pointed at nothing, so every past purchase under that tier lost its "early bird"
+    // attribution — even for tiers the admin didn't touch — and any stock-limit count for the
+    // tier reset to zero (allowing it to be oversold). Upserting by id keeps existing tiers'
+    // ids stable across edits so that history stays linked.
     if (Array.isArray(earlyBirdTiers)) {
-      try { await prisma.earlyBirdTier.deleteMany({ where: { eventOptionId: updatedEventOption.id } }); } catch {}
+      const incomingIds = earlyBirdTiers.filter(t => t && t.id).map(t => t.id);
+      const existingTiers = await prisma.earlyBirdTier.findMany({ where: { eventOptionId: updatedEventOption.id } });
+      for (const et of existingTiers) {
+        if (incomingIds.includes(et.id)) continue;
+        // Never delete a tier that's still attributed on past purchases — just leave it
+        // orphaned from the option's active tier list rather than nulling out history.
+        const usageCount = await prisma.registrationOptionTranche.count({ where: { appliedTierId: et.id } });
+        if (usageCount === 0) {
+          try { await prisma.earlyBirdTier.delete({ where: { id: et.id } }); } catch {}
+        }
+      }
       for (let i = 0; i < earlyBirdTiers.length; i++) {
         const t = earlyBirdTiers[i];
         if (!t || !t.deadline || (t.price === undefined || t.price === null)) continue;
         const deadline = new Date(t.deadline);
         const p = parseFloat(t.price);
         if (!(deadline instanceof Date) || isNaN(deadline.getTime()) || !(p >= 0)) continue;
-        await prisma.earlyBirdTier.create({
-          data: {
-            id: require('uuid').v4(),
-            eventOptionId: updatedEventOption.id,
-            variantId: t.variantId || null,
-            deadline,
-            price: p,
-            order: typeof t.order === 'number' ? t.order : i,
-            stockLimit: t.stockLimit ? parseInt(t.stockLimit, 10) : 0,
-          }
-        });
+        const data = {
+          eventOptionId: updatedEventOption.id,
+          variantId: t.variantId || null,
+          deadline,
+          price: p,
+          order: typeof t.order === 'number' ? t.order : i,
+          stockLimit: t.stockLimit ? parseInt(t.stockLimit, 10) : 0,
+        };
+        if (t.id) {
+          await prisma.earlyBirdTier.upsert({
+            where: { id: t.id },
+            update: data,
+            create: { id: t.id, ...data },
+          });
+        } else {
+          await prisma.earlyBirdTier.create({ data: { id: require('uuid').v4(), ...data } });
+        }
       }
     }