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 '';