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/generateInvoice.server.tsx

@@ -131,6 +131,7 @@ export async function generateInvoice(
 
   const version = latest ? latest.version + 1 : 1;
   const totalsJson = JSON.stringify(viewModel.totals);
+  const paymentStatus = viewModel.paymentStatus;
   const customerJson = JSON.stringify({
     recipient: viewModel.recipient,
     isB2B: viewModel.isB2B,
@@ -148,6 +149,7 @@ export async function generateInvoice(
           pdfUrl: upload.url,
           totalsJson,
           customerJson,
+          paymentStatus,
           issuedAt: new Date(),
           status: "issued",
           lastError: "",
@@ -167,6 +169,7 @@ export async function generateInvoice(
           pdfUrl: upload.url,
           totalsJson,
           customerJson,
+          paymentStatus,
           status: "issued",
         },
       });