Include lead image in feed item descriptions

The API already returns image_affiche_scale/image_full_scale for every
item (news and events alike), no extra request needed — just wasn't
being used. Embedded as an <img> at the top of the HTML description so
feed readers display it inline with the article.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

php/actualites.php

@@ -37,7 +37,7 @@ function build_news_feed(): string
             'link' => SITE . '/actualites/' . slugify($title) . '?u=' . rawurlencode($uid),
             'guid' => $uid,
             'pubDate' => $item['effective'] ?? $item['created'] ?? null,
-            'description' => full_body_html($item),
+            'description' => full_body_html($item, $title),
         ];
     }
     return build_rss(