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/docs/2_customization/00_programmatic-content.md

@@ -0,0 +1,84 @@
+---
+title: Programmatic Content
+intro: Set default SEO values from page models
+---
+
+Sometimes you want SEO fields to default to values from other fields, or generate them from code. A common example is using a plugin like [kirby-paparazzi](https://github.com/tobimori/kirby-paparazzi) to generate OG images for every page.
+
+Add a `metaDefaults` method to a [page model](https://getkirby.com/docs/guide/templates/page-models). It returns an array of meta tag names mapped to their values. These defaults apply through the [Meta Cascade](0_getting-started/1_your-first-meta-tags) when no editor override exists.
+
+```php
+<?php
+// site/models/article.php
+
+use Kirby\Cms\Page;
+
+class ArticlePage extends Page
+{
+  public function metaDefaults(string $lang = null): array
+  {
+    return [
+      'og:image' => "{$this->url()}.png",
+      'og:image:width' => 1230,
+      'og:image:height' => 600,
+      'description' => $this->content($lang)->summary()->value(),
+    ];
+  }
+}
+```
+
+Kirby SEO picks the correct tag syntax from the name. Open Graph keys (starting with `og:`) get `property` and `content` attributes, link keys like `canonical` get `rel` and `href`, and everything else gets `name` and `content`.
+
+## Custom tag attributes
+
+If you need full control over a tag's output, pass an array with `tag` and `attributes`:
+
+```php
+return [
+  // shorthand
+  'description' => 'A page about something',
+
+  // tag with inner content
+  [
+    'tag' => 'title',
+    'content' => 'My Page Title',
+  ],
+
+  // tag with attributes
+  [
+    'tag' => 'meta',
+    'attributes' => [
+      'property' => 'og:image:alt',
+      'content' => "An image of {$this->title()}",
+    ],
+  ],
+
+  // link tag
+  [
+    'tag' => 'link',
+    'attributes' => [
+      'rel' => 'preconnect',
+      'href' => 'https://fonts.googleapis.com',
+    ],
+  ],
+];
+```
+
+## Global defaults via a plugin
+
+Page models only apply to pages with a specific template. If you want to add meta tags to all pages, you can register a `metaDefaults` [page method](https://getkirby.com/docs/reference/plugins/extensions/page-methods) in a plugin:
+
+```php
+<?php
+// site/plugins/my-meta/index.php
+
+Kirby::plugin('my/meta', [
+  'pageMethods' => [
+    'metaDefaults' => function (string $lang = null): array {
+      return [
+        'og:image' => "{$this->url()}.png",
+      ];
+    },
+  ],
+]);
+```