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/agenda.php

@@ -55,7 +55,7 @@ function build_events_feed(): string
         $uid = $item['UID'] ?? $id;
         $meta = format_event_meta($item);
         $description = ($meta !== '' ? '<p>' . htmlspecialchars($meta, ENT_QUOTES, 'UTF-8') . '</p>' : '')
-            . full_body_html($item);
+            . full_body_html($item, $title);
         // Un evenement recurrent partage le meme UID sur toutes ses
         // occurrences (memes UID, dates differentes) : on suffixe le guid
         // avec la date de debut pour rester unique par occurrence, comme