Fix article links and include full content in PHP feeds

Two bugs found by testing real links from the feed:
- Link format was wrong (?news=<internal-id>), landing on the general
  listing instead of the article. Traced the real network call the site
  makes when opening an item and found the actual pattern:
  /actualites/<slug>?u=<UID> (agenda: /agenda/<slug>?u=<UID>), where the
  JS router reads the `u` query param to fetch @results?UID=...&fullobjects=1.
- Description only used the short teaser (description_fr) even though the
  API response already includes the full rich-text body (text_fr.data) and
  attachments (items[].file.download) in the same call — no extra request
  needed, just wasn't being used.

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

php/agenda.php

@@ -51,13 +51,15 @@ function build_events_feed(): string
     $items = [];
     foreach (fetch_all_events() as $item) {
         $id = basename(rtrim($item['@id'] ?? '', '/'));
-        $summary = strip_html($item['description_fr'] ?? $item['description'] ?? null);
+        $title = $item['title_fr'] ?? $item['title'] ?? '(sans titre)';
+        $uid = $item['UID'] ?? $id;
         $meta = format_event_meta($item);
-        $description = implode(' | ', array_filter([$meta, $summary]));
+        $description = ($meta !== '' ? '<p>' . htmlspecialchars($meta, ENT_QUOTES, 'UTF-8') . '</p>' : '')
+            . full_body_html($item);
         $items[] = [
-            'title' => $item['title_fr'] ?? $item['title'] ?? '(sans titre)',
-            'link' => SITE . '/agenda?event=' . rawurlencode($id),
-            'guid' => $item['UID'] ?? $id,
+            'title' => $title,
+            'link' => SITE . '/agenda/' . slugify($title) . '?u=' . rawurlencode($uid),
+            'guid' => $uid,
             'pubDate' => $item['created'] ?? $item['start'] ?? null,
             'description' => $description,
         ];