Wire up the blog section to Payload CMS instead of hardcoded posts

Blog.tsx now fetches the einfach-produktiv tenant's posts from the
shared Payload instance at build/render time (ISR, 60s revalidate),
via a small typed fetch helper in app/lib/payload.ts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/payload.ts

@@ -0,0 +1,57 @@
+const PAYLOAD_URL = process.env.PAYLOAD_URL || "https://payload.mk360.de";
+const TENANT_SLUG = "einfach-produktiv";
+
+export type BlogPost = {
+  id: number;
+  title: string;
+  slug: string;
+  category: string;
+  readTime: number;
+  excerpt: string;
+  thumbnail: string | null;
+};
+
+type PayloadPost = {
+  id: number;
+  title: string;
+  slug: string;
+  category: { name: string } | number | null;
+  readTime: number;
+  excerpt: string;
+  thumbnail: { url: string } | number | null;
+};
+
+export async function getBlogPosts(limit = 3): Promise<BlogPost[]> {
+  const params = new URLSearchParams({
+    "where[tenant.slug][equals]": TENANT_SLUG,
+    sort: "-publishedAt",
+    depth: "2",
+    limit: String(limit),
+  });
+
+  const res = await fetch(`${PAYLOAD_URL}/api/posts?${params}`, {
+    next: { revalidate: 60 },
+  });
+  if (!res.ok) {
+    console.error(`getBlogPosts: Payload returned ${res.status} ${res.statusText}`);
+    return [];
+  }
+
+  const data: { docs?: PayloadPost[] } = await res.json();
+  const docs = Array.isArray(data.docs) ? data.docs : [];
+  return docs.map((post) => ({
+    id: post.id,
+    title: post.title,
+    slug: post.slug,
+    category:
+      typeof post.category === "object" && post.category
+        ? post.category.name
+        : "",
+    readTime: post.readTime,
+    excerpt: post.excerpt,
+    thumbnail:
+      typeof post.thumbnail === "object" && post.thumbnail
+        ? post.thumbnail.url
+        : null,
+  }));
+}