feat(email): per-variant paid/unpaid body templates in settings

The previous single body field could not distinguish 'use the default' from
'save the pre-filled default', so the first settings save locked the hint in
as a custom override for ALL orders — the paid/unpaid default logic never ran.

Give each payment state its own editable body field (emailBodyHtmlDePaid /
emailBodyHtmlEnPaid) with its own empty-fallback. A body submitted unchanged
from its built-in default is stored as empty, so 'use default' survives saves
and future default updates propagate.

prisma/migrations/20260824010000_add_paid_email_templates/migration.sql

@@ -0,0 +1,3 @@
+-- AlterTable
+ALTER TABLE "ShopSettings" ADD COLUMN "emailBodyHtmlDePaid" TEXT NOT NULL DEFAULT '';
+ALTER TABLE "ShopSettings" ADD COLUMN "emailBodyHtmlEnPaid" TEXT NOT NULL DEFAULT '';

prisma/schema.prisma

@@ -94,10 +94,14 @@ model ShopSettings {
   smtpReplyTo       String   @default("")
 
   // Email templates (HTML, with {{var}} placeholders). Empty = use defaults.
+  // The Body fields are the UNPAID (outstanding) variant; the *Paid fields are
+  // sent for orders that are already settled (paid/refunded/voided).
   emailSubjectDe    String   @default("")
   emailBodyHtmlDe   String   @default("")
   emailSubjectEn    String   @default("")
   emailBodyHtmlEn   String   @default("")
+  emailBodyHtmlDePaid String @default("")
+  emailBodyHtmlEnPaid String @default("")
 
   // Automations (webhook-driven, as a fallback to Shopify Flow which only
   // exposes custom-app actions on Plus stores).