feat: prompts admin-only, add isOpen field to Prompt model

prisma/schema.prisma

@@ -169,6 +169,7 @@ model Prompt {
   description String   @db.Text
   content     String   @db.Text
   tags        String[]
+  isOpen      Boolean  @default(false)
   createdAt   DateTime @default(now())
   updatedAt   DateTime @updatedAt
 }