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/actualites.php

@@ -44,7 +44,8 @@ function build_news_feed(): string
         'Silly.be - Actualités',
         SITE . '/actualites',
         'Actualités de la commune de Silly (Belgique)',
-        $items
+        $items,
+        'https://sillycone-village.be/silly-rss/actualites.php'
     );
 }