feat(images): convert all images to WebP with 87% size reduction

- Convert 34 images (PNG/JPG) to WebP at quality 80
- Total savings: 21.6 MB → 2.8 MB (87% reduction)
- Add <picture> elements with WebP source + original fallback
- Add loading=lazy to all below-the-fold images
- Update lightbox to serve WebP images with error fallback

nginx.conf

@@ -5,7 +5,20 @@ server {
     root /usr/share/nginx/html;
     index haus-schleusingen.html;
 
+    # Gzip aktivieren
+    gzip on;
+    gzip_types text/css application/javascript image/svg+xml application/json text/xml;
+    gzip_min_length 256;
+    gzip_vary on;
+
     location / {
         try_files $uri $uri/ /haus-schleusingen.html;
     }
-}
+
+    # Lange Cache-Dauer für Bilder und statische Assets
+    location ~* \.(jpg|jpeg|png|webp|gif|ico|svg|css|js|woff2?)$ {
+        expires 30d;
+        add_header Cache-Control "public, immutable";
+        access_log off;
+    }
+}