feat(issued-orders): editable PDF heading 'Text objednavky (na PDF)' (order_text)

New nullable issued_orders.order_text column (migration applied to dev+test). The heading above the PDF items table is now editable per order, mirroring invoices' billing_text: empty falls back to the default, which is now 'Objednavame si u Vas:' per user wording. Field sits above Dodaci podminky in the detail form; persisted via create + update strFields (both schemas - the invoices Update-schema gap is not repeated). Tests: persistence round-trip incl. null-clears, PDF custom heading + default fallback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

prisma/migrations/20260610121316_issued_orders_order_text/migration.sql

@@ -0,0 +1,3 @@
+-- AlterTable
+ALTER TABLE `issued_orders` ADD COLUMN `order_text` VARCHAR(500) NULL;
+

prisma/schema.prisma

@@ -381,6 +381,7 @@ model issued_orders {
   delivery_terms  String?              @db.VarChar(500)
   payment_terms   String?              @db.VarChar(500)
   issued_by       String?              @db.VarChar(255)
+  order_text      String?              @db.VarChar(500)
   notes           String?              @db.Text
   internal_notes  String?              @db.Text
   created_at      DateTime?            @default(now()) @db.DateTime(0)