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/prompts/tasks/og-site-description.php

@@ -0,0 +1,25 @@
+<?php
+
+/** @var \Kirby\Cms\Page $page
+ ** @var \Kirby\Cms\Site $site
+ ** @var string|null $instructions
+ ** @var string|null $edit */
+
+snippet('seo/prompts/introduction', [
+	'instructions' => $instructions ?? null,
+	'edit' => $edit ?? null
+]); ?>
+
+<task>
+	Create a useful GLOBAL open graph description for this site <site-title><?= $site->title()->value() ?>.</site-title>
+	This description will be shown on social media platforms like Facebook, WhatsApp and LinkedIn.
+	This description is meant as FALLBACK for when the page does not have a meta description itself.
+	This description should be unique and relevant to the site's content.
+
+	The entire meta description SHOULD be between 120 and 158 characters long.
+
+	You'll receive the content of the home page as well as any meta tags that are already set below.
+</task>
+
+<?php snippet('seo/prompts/site-meta', ['currentField' => 'ogDescription']);
+snippet('seo/prompts/content'); ?>