feat(payments): add upsertPaymentFromHelcim helper with idempotent insert

Takes a Member doc + a normalized Helcim transaction and inserts a
Payment row if helcimTransactionId is unseen. Maps helcim status
paid→success, refunded→refunded, failed→failed; skips 'other'.

opts.paymentType overrides the cadence fallback for mid-flight cadence
changes. opts.sendConfirmation triggers a Resend payment-confirmation
email ONLY on new inserts — swallows send failures so email trouble
cannot break the upstream payment flow.

The Resend template lives in server/emails/paymentConfirmation.js. It
is CRA-safe (charity name + 'not an official donation receipt / tax
receipts available later in 2026' disclaimer) so it can be used in
either Task 8 branch without copy changes.

server/emails/paymentConfirmation.js

@@ -0,0 +1,16 @@
+export const paymentConfirmationEmail = ({ member, amount, paymentDate, transactionId }) => ({
+  from: 'Ghost Guild <ghostguild@babyghosts.org>',
+  to: member.email,
+  subject: 'Payment confirmation — Ghost Guild',
+  text: `Hi ${member.name || 'there'},
+
+We received your payment of $${Number(amount).toFixed(2)} CAD on ${new Date(paymentDate).toLocaleDateString('en-CA')}.
+
+Baby Ghosts Studio Development Fund (BN 788709350 RR 0001)
+Helcim transaction ID: ${transactionId}
+
+This is a payment confirmation, not an official donation receipt for Canadian tax purposes. Tax receipts for eligible members will be available starting later in 2026.
+
+Thanks for being part of Ghost Guild.
+`
+})

server/utils/payments.js

@@ -0,0 +1,63 @@
+import { Resend } from 'resend'
+import Payment from '../models/payment.js'
+import { paymentConfirmationEmail } from '../emails/paymentConfirmation.js'
+
+const resend = new Resend(process.env.RESEND_API_KEY)
+
+function mapStatus(helcimStatus) {
+  if (helcimStatus === 'paid') return 'success'
+  if (helcimStatus === 'refunded') return 'refunded'
+  if (helcimStatus === 'failed') return 'failed'
+  return null
+}
+
+export async function upsertPaymentFromHelcim(memberDoc, helcimTx, opts = {}) {
+  const status = mapStatus(helcimTx?.status)
+  if (!status) return { created: false, payment: null }
+
+  const existing = await Payment.findOne({ helcimTransactionId: helcimTx.id })
+  if (existing) return { created: false, payment: existing }
+
+  const paymentType = opts.paymentType || memberDoc.billingCadence
+
+  const fields = {
+    memberId: memberDoc._id,
+    helcimTransactionId: helcimTx.id,
+    helcimCustomerId: memberDoc.helcimCustomerId || null,
+    helcimSubscriptionId: memberDoc.helcimSubscriptionId || null,
+    amount: helcimTx.amount,
+    currency: helcimTx.currency || 'CAD',
+    paymentDate: helcimTx.date ? new Date(helcimTx.date) : new Date(),
+    paymentType,
+    status,
+    rawHelcim: helcimTx
+  }
+
+  let payment
+  try {
+    payment = await Payment.create(fields)
+  } catch (err) {
+    if (err?.code === 11000) {
+      const racer = await Payment.findOne({ helcimTransactionId: helcimTx.id })
+      return { created: false, payment: racer }
+    }
+    throw err
+  }
+
+  if (opts.sendConfirmation) {
+    try {
+      await resend.emails.send(
+        paymentConfirmationEmail({
+          member: memberDoc,
+          amount: fields.amount,
+          paymentDate: fields.paymentDate,
+          transactionId: fields.helcimTransactionId
+        })
+      )
+    } catch (err) {
+      console.error('[payments] failed to send payment confirmation email:', err?.message || err)
+    }
+  }
+
+  return { created: true, payment }
+}