Add auto-updating sitemap.xml and robots.txt

Sitemap combines hardcoded static routes with Payload's Pages and
published Posts collections (hourly ISR revalidation), so new CMS
pages/blog posts appear without a code change. Excludes noindex
transactional routes (checkout, cart, bestellbestaetigung,
newsletter-confirmed, konto/*) to avoid Search Console warnings.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013Eg6h91yngXmSnM51wxXM8

app/robots.ts

@@ -0,0 +1,16 @@
+import type { MetadataRoute } from "next";
+
+const SITE_URL = "https://einfach-produktiv.mk360.de";
+
+export default function robots(): MetadataRoute.Robots {
+  return {
+    rules: [
+      {
+        userAgent: "*",
+        allow: "/",
+        disallow: ["/checkout", "/cart", "/konto", "/bestellbestaetigung", "/newsletter-confirmed", "/email-preview", "/company-settings-preview", "/api"],
+      },
+    ],
+    sitemap: `${SITE_URL}/sitemap.xml`,
+  };
+}