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/routes/app.settings.tsx

@@ -421,7 +421,12 @@ export default function SettingsRoute() {
           <s-stack direction="block" gap="base">
             <s-paragraph>
               These templates are used when sending the invoice PDF by email.
-              Leave a field empty to fall back to the built-in default.
+              Leave a field empty to fall back to the built-in default. The
+              built-in default has two variants per language: orders that are
+              already paid are sent a body <em>without</em> the bank-transfer
+              reference hint, while outstanding (unpaid) orders include it.
+              A custom template you set here is used for both paid and unpaid
+              orders — the editor below shows the unpaid default.
             </s-paragraph>
 
             <Field