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/1_features/05_alt-texts.md

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+---
+title: Alt Text Field
+intro: Structured alt text for images, with AI generation and a decorative toggle
+---
+
+Every image on the web needs an `alt` attribute. Images that convey meaning need descriptive text. Decorative images need an empty `alt=""`, which tells screen readers to skip them entirely. Getting this wrong hurts accessibility.
+
+Kirby SEO provides a dedicated `alt-text` field that handles both cases. It stores structured data instead of a plain string, so your templates always render the correct HTML attributes.
+
+## Adding the field
+
+Add a `alt-text` field to any file blueprint:
+
+```yaml
+# site/blueprints/files/image.yml
+fields:
+  alt:
+    type: alt-text
+    label: Alt Text
+```
+
+Editors see a text input with a toggle. The toggle marks an image as decorative: when active, the text input disappears because decorative images don't need a description.
+
+## AI generation
+
+If [AI Assist](1_features/04_ai-assist) is configured, the field shows **Generate** and **Customize** buttons. The AI sees the actual image and writes alt text based on it, the filename, and the page context. Results stream in word by word and can be stopped early.
+
+You can disable AI for a specific field by setting `ai: false` in the blueprint.
+
+### Auto-generation on upload
+
+Set `autogenerate: true` to generate alt text automatically when an image is uploaded:
+
+```yaml
+alt:
+  type: alt-text
+  autogenerate: true
+```
+
+By default, this runs synchronously during the upload. For better performance, you can offload it to a background queue. See [Background Processing](2_customization/10_background-processing) for setup. On multilingual sites, a single AI call generates alt text for all languages at once.
+
+## Using alt text in templates
+
+The plugin registers a `toAltText()` field method that returns an `AltText` object. Use its `toAttr()` method to get the correct HTML attributes, then spread them into your image helper:
+
+```php
+<?= Html::img($file->url(), [
+  'width' => $file->width(),
+  'height' => $file->height(),
+  ...$file->alt()->toAltText()->toAttr(),
+]) ?>
+// <img alt="A dog playing fetch" src="..." width="..." height="...">
+
+// decorative image:
+// <img alt="" src="..." width="..." height="...">
+
+The field also works with plain string values from existing `alt` fields. If you migrate from a regular text field, `toAltText()` treats the old value as manual alt text.