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/blueprints/pages/impact-media.yml

@@ -0,0 +1,16 @@
+title: Impact médiatique
+
+fields:
+  linkedInvestigation:
+    label: Enquête liée
+    type: pages
+    query: page('enquetes').children.listed
+    multiple: false
+    layout: list
+    text: "{{ page.title }}"
+    info: "{{ page.created }}"
+  entries:
+    label: Articles et reprises
+    type: entries
+    field:
+      type: url