Product JSON-LD: add sku + gallery images, single-source the meta description

sku was tracked in Payload but never exposed to the frontend at all —
added to the Product type and wired into buildProductSchema (a real
Schema.org property Google's rich-result validator checks for).
image is now an array (main + gallery) when a product has gallery
photos, instead of always just the one main image.

/der-eine's page metadata description was a separately hardcoded
string that had drifted from Products.description (the one JSON-LD/
cart/checkout actually use) — now reads from the live product via
generateMetadata, same pattern /tasse-die-pause already uses. Updated
Payload's own description field to the more product-descriptive text
that used to live only in that hardcoded string.

app/lib/structuredData.ts

@@ -54,8 +54,9 @@ export function buildProductSchema(product: Product, url: string, seller: Compan
     "@type": "Product",
     name: product.name,
     description: product.description,
-    image: product.image,
+    image: product.gallery.length > 0 ? [product.image, ...product.gallery] : product.image,
     url,
+    ...(product.sku ? { sku: product.sku } : {}),
     // No reviews/ratings system exists yet — `aggregateRating` is
     // optional in the spec and deliberately omitted rather than faked;
     // add it here once real reviews exist, not before.