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)

prisma/schema.prisma

@@ -141,7 +141,12 @@ model Invoice {
 
   // Snapshots (JSON strings on sqlite)
   totalsJson        String   @default("{}")
-  customerJson      String   @default("{}")
+  customerJson      String   @default("")
+
+  // Payment state at generation time ("paid" | "partial" | "unpaid" |
+  // "refunded" | "voided"). Used to pick the paid/unpaid email template at
+  // send time. Empty for legacy rows → treated as "unpaid".
+  paymentStatus     String   @default("")
 
   // Lifecycle
   issuedAt          DateTime @default(now())