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)

tests/email-templates.test.ts

@@ -0,0 +1,39 @@
+import { strict as assert } from "node:assert";
+import { describe, it } from "node:test";
+
+import {
+  DEFAULT_EMAIL_BODY_DE_PAID,
+  DEFAULT_EMAIL_BODY_DE_UNPAID,
+  DEFAULT_EMAIL_BODY_EN_PAID,
+  DEFAULT_EMAIL_BODY_EN_UNPAID,
+} from "../app/services/invoice/emailTemplates";
+
+describe("invoice email templates (paid vs unpaid)", () => {
+  const HINT_EN = /bank transfer/i;
+  const HINT_DE = /Überweisung/i;
+
+  it("unpaid defaults keep the bank-transfer reference hint", () => {
+    assert.match(DEFAULT_EMAIL_BODY_EN_UNPAID, HINT_EN);
+    assert.match(DEFAULT_EMAIL_BODY_EN_UNPAID, /\{\{invoiceNumber\}\}/);
+    assert.match(DEFAULT_EMAIL_BODY_DE_UNPAID, HINT_DE);
+    assert.match(DEFAULT_EMAIL_BODY_DE_UNPAID, /\{\{invoiceNumber\}\}/);
+  });
+
+  it("paid defaults omit the bank-transfer reference hint", () => {
+    assert.doesNotMatch(DEFAULT_EMAIL_BODY_EN_PAID, HINT_EN);
+    assert.doesNotMatch(DEFAULT_EMAIL_BODY_DE_PAID, HINT_DE);
+  });
+
+  it("paid and unpaid variants otherwise share header/footer structure", () => {
+    for (const tpl of [
+      DEFAULT_EMAIL_BODY_EN_PAID,
+      DEFAULT_EMAIL_BODY_EN_UNPAID,
+      DEFAULT_EMAIL_BODY_DE_PAID,
+      DEFAULT_EMAIL_BODY_DE_UNPAID,
+    ]) {
+      assert.match(tpl, /cid:invoice-logo/);
+      assert.match(tpl, /\{\{companyName\}\}/);
+      assert.match(tpl, /\{\{shopEmail\}\}/);
+    }
+  });
+});