feat: intégration plugin Kirby SEO

- Ajout de tobimori/kirby-seo via Composer
- snippet('seo/head') dans header.php (remplace les meta manuels)
- snippet('seo/schemas') dans footer.php pour JSON-LD
- Onglet SEO ajouté dans site.yml et tous les blueprints de pages
- Configuration SEO dans config.php (sitemap, robots, canonicalBase TODO)

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

site/plugins/kirby-seo/snippets/head.php

@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @var \Kirby\Cms\Page $page
+ */
+
+use Kirby\Cms\Html;
+
+$tags = $page->metadata()->snippetData();
+
+// if we're using slots, the user wants to output priority tags such as <title>
+// before their stylesheet, script, etc. tags
+if (isset($slot)) {
+	foreach (array_filter($tags, fn ($tag) => $tag['priority']) as $tag) {
+		echo Html::tag($tag['tag'], $tag['content'] ?? null, $tag['attributes'] ?? []) . PHP_EOL;
+	}
+
+	echo $slot;
+
+	$tags = array_filter($tags, fn ($tag) => !$tag['priority']);
+}
+
+// then output other tags as normal
+// this is unfiltered if slots is not set.
+foreach ($tags as $tag) {
+	echo Html::tag($tag['tag'], $tag['content'] ?? null, $tag['attributes'] ?? []) . PHP_EOL;
+}