feat(seo): add meta description, canonical, Open Graph and Schema.org JSON-LD

- Add meta description with property details
- Add canonical URL (haus-schleusingen.de placeholder, pending Martin's confirmation)
- Add Open Graph tags (og:type, og:title, og:description, og:image, og:url, og:locale, og:site_name)
- Add Schema.org RealEstateListing structured data (JSON-LD)
- Optimize title tag for SEO
- Create robots.txt with sitemap reference

Ref #16

robots.txt

@@ -0,0 +1,3 @@
+User-agent: *
+Allow: /
+Sitemap: https://haus-schleusingen.de/haus-schleusingen.html