Lazy-load the post detail hero image

It's below the fold for most viewports, so deferring the fetch cuts
initial-paint bytes without affecting readers who scroll.

Author: julioest

templates/news/v3/detail.html

@@ -45,7 +45,7 @@
 
       {% if object.image %}
         <figure class="post-detail__figure">
-          <img src="{{ object.image.url }}" alt="" class="post-detail__image">
+          <img src="{{ object.image.url }}" alt="" class="post-detail__image" loading="lazy">
         </figure>
       {% endif %}