fix(email): sibling billing emails follow customer language too

Addresses the PR #763 review: the invoice_sent / storno_issued / payment-check
/ paid-admin-notification emails share the identical event-first language bug
and never set __language, so a German customer on an English-gallery event got
an English email body with German-formatted amounts.

Each call site already computes the locale it formats amounts in, so this is a
one-liner per call — the body language now matches the amount formatting:
- invoice_sent, storno_issued (sending.js) -> __language: ctx.locale
- payment-check, invoice_paid_admin_notification (payments.js) -> __language: locale

Leak-safe (no template references {{__language}}) and falls back to the
existing event-first resolution when unset, per the mechanism added in #763.

backend/src/services/invoice/payments.js

@@ -185,6 +185,8 @@ async function queueInvoicePaidAdminNotification({
         || [customer?.first_name, customer?.last_name].filter(Boolean).join(' ')
         || customer?.email || '',
       event_name: invoice.event_name || '',
+      // Keep the body language consistent with the locale-formatted amounts.
+      __language: locale,
       total_amount: formatMajor(invoice.total_amount_minor, invoice.currency, locale),
       paid_amount: formatMajor(paidTotalMinor, invoice.currency, locale),
       payment_method: paymentMethod || '',
@@ -285,6 +287,9 @@ async function queuePaymentCheckEmail(invoiceId, { skipThrottle = false } = {})
         || [customer?.first_name, customer?.last_name].filter(Boolean).join(' ')
         || customer?.email || '',
       event_name: invoice.event_name || '',
+      // Keep the body language consistent with the locale the amounts are
+      // formatted in, instead of event-first resolution (admin-facing gate).
+      __language: locale,
       due_date: formatShortDate(invoice.due_date),
       total_amount: formatMajor(invoice.total_amount_minor, invoice.currency, locale),
       paid_amount: formatMajor(paidMinor, invoice.currency, locale),

backend/src/services/invoice/sending.js

@@ -127,6 +127,9 @@ async function sendInvoice(id, adminId) {
     installment_label: invoice.installment_label || '',
     installment_index: invoice.installment_index + 1,
     installment_total: invoice.installment_total,
+    // Send in the customer's language (matches the ctx.locale-formatted amounts
+    // above) rather than the event-first default resolution.
+    __language: ctx.locale,
     cc: invoiceCc,
     attachments: [{
       filename: `${invoice.invoice_number}.pdf`,
@@ -368,6 +371,8 @@ async function sendStorno(stornoId, adminId) {
     original_issue_date: originalRow?.issue_date ? formatShortDate(originalRow.issue_date) : '',
     customer_name: customer.display_name || customer.first_name || customer.email.split('@')[0],
     total_amount: formatMajor(Math.abs(storno.total_amount_minor), storno.currency, ctx.locale),
+    // Match the customer's language (as with the ctx.locale-formatted amount).
+    __language: ctx.locale,
     cc: stornoCc,
     attachments: [{
       filename: `${storno.invoice_number}.pdf`,