feat(email): split invoice email into paid/unpaid variants

Persist paymentStatus on the Invoice at generation time and use it at send
time to pick the default email body. Paid orders (paid/refunded/voided) now
get a body without the bank-transfer reference hint; outstanding orders keep
it. Custom merchant templates still override the defaults for both cases.

- Add Invoice.paymentStatus column + migration
- Split DEFAULT_EMAIL_BODY_{DE,EN} into _PAID/_UNPAID variants
- Persist paymentStatus in generateInvoice; select default in email.server
- Document behaviour on the settings page
- Add email-templates test (paid omits hint, unpaid keeps it)

app/services/invoice/email.server.ts

@@ -3,10 +3,12 @@ import type { Transporter } from "nodemailer";
 import type { ShopSettings } from "@prisma/client";
 
 import db from "../../db.server";
-import { getStrings, pickLanguage } from "./i18n";
+import { pickLanguage } from "./i18n";
 import {
-  DEFAULT_EMAIL_BODY_DE,
-  DEFAULT_EMAIL_BODY_EN,
+  DEFAULT_EMAIL_BODY_DE_PAID,
+  DEFAULT_EMAIL_BODY_DE_UNPAID,
+  DEFAULT_EMAIL_BODY_EN_PAID,
+  DEFAULT_EMAIL_BODY_EN_UNPAID,
   DEFAULT_EMAIL_SUBJECT_DE,
   DEFAULT_EMAIL_SUBJECT_EN,
 } from "./emailTemplates";
@@ -74,7 +76,6 @@ export async function sendInvoiceEmail(
   // rendered in, so the email matches the attachment. Caller can still
   // override via `customerLocale` if they really want a different language.
   const language = pickLanguage(args.customerLocale ?? invoice.language ?? settings.defaultLanguage);
-  const t = getStrings(language);
   const customer = parseCustomer(invoice.customerJson);
   const totals = parseTotals(invoice.totalsJson);
   const vars = buildTemplateVars({
@@ -90,9 +91,26 @@ export async function sendInvoiceEmail(
     (language === "en" ? DEFAULT_EMAIL_SUBJECT_EN : DEFAULT_EMAIL_SUBJECT_DE);
   const subject = renderTemplate(customSubject, vars, { html: false });
 
+  // Pick the default body variant for the invoice's payment state: orders that
+  // are already settled (paid / refunded / voided) get a body WITHOUT the
+  // bank-transfer reference hint; outstanding orders (unpaid / partial) keep
+  // it. Legacy rows have an empty paymentStatus → treated as unpaid.
+  // A merchant-supplied custom template always wins over the defaults.
+  const isPaid =
+    invoice.paymentStatus === "paid" ||
+    invoice.paymentStatus === "refunded" ||
+    invoice.paymentStatus === "voided";
+  const defaultBody =
+    language === "en"
+      ? isPaid
+        ? DEFAULT_EMAIL_BODY_EN_PAID
+        : DEFAULT_EMAIL_BODY_EN_UNPAID
+      : isPaid
+        ? DEFAULT_EMAIL_BODY_DE_PAID
+        : DEFAULT_EMAIL_BODY_DE_UNPAID;
   const customBodyHtml =
     (language === "en" ? settings.emailBodyHtmlEn : settings.emailBodyHtmlDe) ||
-    (language === "en" ? DEFAULT_EMAIL_BODY_EN : DEFAULT_EMAIL_BODY_DE);
+    defaultBody;
   const body = renderHtmlBody(renderTemplate(customBodyHtml, vars));
 
   // If the rendered body references the inline logo, attach it.