feat: Improve bulk category update functionality with validation and clearer logic

backend/src/products/products.service.ts

@@ -389,12 +389,15 @@ export class ProductsService {
 
   async bulkUpdate(ids: number[], data: { categoryId?: number | null }) {
     const updateData: Record<string, any> = {};
-    if ('categoryId' in data) {
+    if (data.categoryId !== undefined) {
       updateData.categoryId = data.categoryId;
     }
     if (Object.keys(updateData).length === 0) return { updated: 0 };
-    await this.prisma.product.updateMany({ where: { id: { in: ids } }, data: updateData });
-    return { updated: ids.length };
+    const result = await this.prisma.product.updateMany({
+      where: { id: { in: ids } },
+      data: updateData,
+    });
+    return { updated: result.count };
   }
 
   async findUncategorized(): Promise<{ id: number; name: string; canonicalName: string | null }[]> {