feat: impact-media page type with OG scraping and cache

- Add impact-media blueprint (entries field + linked investigation)
- Add minimal impact-media template
- Refactor card-open-graph snippet: accepts $url param, Kirby cache (6h TTL), decode HTML entities, empty alt on images
- Update impacts.yml to allow impact-media pages
- Render impact-media in investigation aside with dynamic count + details/summary
- Add OG cache config in config.php
- CSS formatting fixes (body, card-block-small, category)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

site/config/config.php

@@ -76,6 +76,10 @@ return [
   ],
   'tobimori.seo.canonicalBase' => 'https://www.index.ngo',
 
+  'cache' => [
+    'og' => true,
+  ],
+
   'routes' => [
     require(__DIR__ . '/routes/newsletter.php'),
   ],