feat(SEO): add price validity date to category JSON-LD

- Introduced a priceValidUntil field in the category JSON-LD to indicate the validity period of product prices, set to three months from the current date.
- This enhancement improves the structured data for SEO, providing clearer information about pricing timelines.

prerender/seo/category.cjs

@@ -1,6 +1,11 @@
 const generateCategoryJsonLd = (category, products = [], baseUrl, config) => {
   const categoryUrl = `${baseUrl}/Kategorie/${category.seoName}`;
 
+  // Calculate price valid date (current date + 3 months)
+  const priceValidDate = new Date();
+  priceValidDate.setMonth(priceValidDate.getMonth() + 3);
+  const priceValidUntil = priceValidDate.toISOString().split("T")[0];
+
   const jsonLd = {
     "@context": "https://schema.org/",
     "@type": "CollectionPage",
@@ -57,6 +62,7 @@ const generateCategoryJsonLd = (category, products = [], baseUrl, config) => {
             url: `${baseUrl}/Artikel/${product.seoName}`,
             price: product.price && !isNaN(product.price) ? product.price.toString() : "0.00",
             priceCurrency: config.currency,
+            priceValidUntil: priceValidUntil,
             availability: product.available
               ? "https://schema.org/InStock"
               : "https://schema.org/OutOfStock",