feat: add isPrivate field to Product model and implement private product creation and retrieval

backend/prisma/migrations/20260501000000_add_product_is_private/migration.sql

@@ -0,0 +1,2 @@
+-- Privata produkter: skapade av användare, synliga bara för ägaren
+ALTER TABLE `Product` ADD COLUMN `isPrivate` BOOLEAN NOT NULL DEFAULT false;

backend/prisma/schema.prisma

@@ -52,6 +52,7 @@ model Product {
   userProducts      UserProduct[]
   categoryId        Int?
   categoryRef       Category?       @relation(fields: [categoryId], references: [id], onDelete: SetNull)
+  isPrivate         Boolean         @default(false)
 }
 
 model Category {