Fix W3C validator errors: duplicate guids and missing atom:link self

- Recurring events share one UID across all occurrences (confirmed via
  the API: same UID, different start dates), which produced duplicate
  <guid> values the RSS spec forbids. Suffix the guid with the
  occurrence's start date to make it unique per occurrence.
- Add atom:link rel="self" to the channel, per the validator's
  recommendation, in both the PHP and Python implementations.

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

php/agenda.php

@@ -56,10 +56,14 @@ function build_events_feed(): string
         $meta = format_event_meta($item);
         $description = ($meta !== '' ? '<p>' . htmlspecialchars($meta, ENT_QUOTES, 'UTF-8') . '</p>' : '')
             . full_body_html($item);
+        // 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
+        // l'exige la spec RSS.
         $items[] = [
             'title' => $title,
             'link' => SITE . '/agenda/' . slugify($title) . '?u=' . rawurlencode($uid),
-            'guid' => $uid,
+            'guid' => $uid . '-' . ($item['start'] ?? ''),
             'pubDate' => $item['created'] ?? $item['start'] ?? null,
             'description' => $description,
         ];
@@ -68,7 +72,8 @@ function build_events_feed(): string
         'Silly.be - Agenda',
         SITE . '/agenda',
         "Agenda / événements de la commune de Silly (Belgique)",
-        $items
+        $items,
+        'https://sillycone-village.be/silly-rss/agenda.php'
     );
 }