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/templates/impact-media.php

@@ -0,0 +1,47 @@
+<?php snippet('header') ?>
+
+<main class="main__single">
+
+<header class="page__header">
+    <p class="page__type">Impact</p>
+    <p class="page__category">Médiatique</p>
+    <h2 class="page__title"><?= $page->title()->esc() ?></h2>
+</header>
+
+<?php
+$entries = $page->entries()->toEntries();
+$investigation = $page->linkedInvestigation()->toPages()->first();
+?>
+
+<?php if ($entries->isNotEmpty()): ?>
+<div class="page__content">
+    <?php foreach ($entries as $entry): ?>
+        <?php snippet('card-open-graph', ['url' => $entry->value()]) ?>
+    <?php endforeach ?>
+</div>
+<?php endif ?>
+
+<?php if ($investigation): ?>
+<aside class="page__aside" id="related-investigations">
+    <h3 class="aside__title">Enquête liée</h3>
+
+    <article class="card--article-small">
+        <?php if ($cover = $investigation->cover()->toFile()): ?>
+        <figure>
+            <img src="<?= $cover->url() ?>" alt="<?= $investigation->title()->esc() ?>">
+        </figure>
+        <?php endif ?>
+        <div class="content">
+            <p class="type">Enquête</p>
+            <h4 class="title"><a href="<?= $investigation->url() ?>"><?= $investigation->title()->esc() ?></a></h4>
+        </div>
+        <button class="btn--go-to"><a href="<?= $investigation->url() ?>"><?= svg('assets/icons/arrow-left.svg') ?></a></button>
+        <a class="link-block" href="<?= $investigation->url() ?>" aria-hidden="true"></a>
+    </article>
+</aside>
+<?php endif ?>
+
+</main>
+
+<?php snippet('bottom-bar') ?>
+<?php snippet('footer') ?>