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/payload.ts

@@ -182,6 +182,10 @@ export type Product = {
   numericId: number;
   name: string;
   description: string;
+  // Not shown anywhere in the UI today — only consumed by
+  // buildProductSchema (JSON-LD's Product.sku), a real Schema.org property
+  // Google's rich-result validator looks for.
+  sku: string | null;
   price: number;
   compareAtPrice: number | null;
   image: string;
@@ -264,6 +268,7 @@ type PayloadProduct = {
   name: string;
   slug: string;
   description: string | null;
+  sku: string | null;
   price: number;
   compareAtPrice: number | null;
   image: { url: string } | number | null;
@@ -330,6 +335,7 @@ export function mapPayloadProduct(product: PayloadProduct): Product {
     numericId: product.id,
     name: product.name,
     description: product.description ?? "",
+    sku: product.sku || null,
     price: product.price,
     compareAtPrice: product.compareAtPrice ?? null,
     image: typeof product.image === "object" && product.image ? product.image.url : "",