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/01_meta-cascade.md

@@ -0,0 +1,95 @@
+---
+title: Meta Cascade
+intro: Understand how meta values are resolved across multiple levels
+---
+
+Kirby SEO is built with a cascading approach. Meta tags can be defined on multiple levels, and they are merged based on priority. If a value is empty on one level, it falls through to the next. This is how the plugin forms the final metadata for every page.
+
+The default cascade, in order of priority:
+
+1. **Page fields** (`fields`) -- Values the editor enters in the page's SEO blueprint fields. This is the highest priority: if an editor sets a meta description, it always wins.
+
+2. **Programmatic** (`programmatic`) -- Values returned by `metaDefaults()` in [page models](2_customization/00_programmatic-content). Use this for computed defaults like generated OG images or descriptions derived from other fields.
+
+3. **Parent** (`parent`) -- Inherited values from the parent page. If a parent page has "inherit settings" enabled for a field, its children pick up those values. Useful for giving all blog posts the same title template, for example.
+
+4. **Fallback fields** (`fallbackFields`) -- Falls back to meta field values for Open Graph tags. If no `ogDescription` is set, the page's `metaDescription` is used instead.
+
+5. **Site** (`site`) -- Global values from the site's SEO blueprint fields. These apply to all pages that don't have their own value set at a higher level.
+
+6. **Options** (`options`) -- The final fallback, defined in the plugin's config defaults. These are the built-in defaults like the title template `{{ title }} - {{ site.title }}`.
+
+## Configuring the cascade
+
+The cascade order is configurable in your `config.php`. You can remove levels, reorder them, or add optional ones:
+
+```php
+<?php
+// site/config/config.php
+
+return [
+  'tobimori.seo' => [
+    'cascade' => [
+      'fields',
+      'programmatic',
+      'parent',
+      'fallbackFields',
+      'site',
+      'options',
+    ],
+  ],
+];
+```
+
+Remove an entry to skip that level entirely. For example, to disable parent inheritance:
+
+```php
+'cascade' => [
+  'fields',
+  'programmatic',
+  'fallbackFields',
+  'site',
+  'options',
+],
+```
+
+## Restore the 1.x behavior
+
+In 1.x, if you set an `ogDescription` at the site level, it applied to every page, even pages that had their own `metaDescription`. The page-specific description never made it into the Open Graph tags.
+
+In 2.x, the `fallbackFields` level sits between `parent` and `site`, so a page's `metaDescription` is used as `ogDescription` before site-wide Open Graph values are reached.
+
+To restore the 1.x behavior, remove `fallbackFields` from the cascade:
+
+```php
+'cascade' => [
+  'fields',
+  'programmatic',
+  'parent',
+  'site',
+  'options',
+],
+```
+
+<details>
+<summary>If you used <code>fallbackFields</code> with additional mappings in 1.x</summary>
+
+In 1.x, `fallbackFields` also mapped `ogTemplate` to `metaTemplate`. If you relied on this, you can restore it by extending the `Meta` class and overriding the `FALLBACK_MAP` constant:
+
+```php
+<?php
+
+use tobimori\Seo\Meta;
+
+class MyMeta extends Meta
+{
+  public const FALLBACK_MAP = [
+    'ogDescription' => 'metaDescription',
+    'ogTemplate' => 'metaTemplate',
+  ];
+}
+```
+
+Then register your class in the config. See [Extending the Plugin](2_customization/11_plugin-extensions) for details.
+
+</details>