feat(Sitemap, SEO): update sitemap generation and SEO configurations

- Enhanced update-sitemap.sh to generate a new sitemap structure with lastmod dates for additional URLs.
- Updated SEO configurations in server.js and seo.js to allow indexing for impressum and datenschutz pages.
- Introduced a list of noindex prefixes to manage SEO settings dynamically based on route paths.
- Added structured FAQ schema in index.html to improve search engine visibility and user engagement.

frontend/public/sitemap.xml

@@ -5,8 +5,20 @@
         http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">
   <url>
     <loc>https://tt-tagebuch.de/</loc>
-    <lastmod>2026-03-18</lastmod>
+    <lastmod>2026-03-27</lastmod>
     <changefreq>weekly</changefreq>
     <priority>1.0</priority>
   </url>
+  <url>
+    <loc>https://tt-tagebuch.de/impressum</loc>
+    <lastmod>2026-03-27</lastmod>
+    <changefreq>yearly</changefreq>
+    <priority>0.3</priority>
+  </url>
+  <url>
+    <loc>https://tt-tagebuch.de/datenschutz</loc>
+    <lastmod>2026-03-27</lastmod>
+    <changefreq>yearly</changefreq>
+    <priority>0.3</priority>
+  </url>
 </urlset>