feat(products): include ownerId in product creation and enforce its requirement

backend/src/products/products.service.ts

@@ -116,7 +116,7 @@ export class ProductsService {
     return product;
   }
 
-  async create(data: CreateProductDto) {
+  async create(data: CreateProductDto, ownerId?: number) {
     const name = data.name.trim();
     const normalizedName = normalizeName(name);
 
@@ -140,8 +140,13 @@ export class ProductsService {
       return existing;
     }
 
+    if (!ownerId) {
+      throw new Error('ownerId är obligatorisk för att skapa en produkt');
+    }
+
     return this.prisma.product.create({
       data: {
+        ownerId,
         name,
         normalizedName,
         canonicalName: name,