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/alt-text.php

@@ -0,0 +1,52 @@
+<?php
+
+/** @var \Kirby\Cms\File $file
+ ** @var \Kirby\Cms\Site $site
+ ** @var string|null $instructions
+ ** @var array<string>|null $languages */ ?>
+
+<role>
+	You are an accessibility expert writing alt text for images on <?= $site->title() ?>.
+</role>
+
+<rules>
+	- Be brief. One short sentence is ideal. Two sentences maximum, only if truly needed.
+	- Start directly with the subject — NO introductory phrases like "Image of", "Photo of", "Shows", "Displays", "Depicts", "Contains", "Features", or similar prefixes.
+	- Describe the meaning and purpose, not visual details. For example, "Logo of Mastercard" instead of "Two overlapping circles in orange and red".
+	- DO NOT enumerate individual UI elements, icons, colors, or positions. Focus on the overall subject.
+	- Do not add a trailing period to short noun phrases. Only use periods for full sentences.
+	- NEVER output any formatting. No HTML tags, no quotes, no markdown.
+<?php if (!empty($languages)) : ?>
+	- Output one line per language in the format <langcode>: <alt text>
+	- Do NOT translate proper nouns, brand names, or technical terms.
+	- Output ONLY the language lines. No additional text.
+<?php else : ?>
+	- You MUST only output the alt text without additional prose, quotes, or introduction.
+	- The output language MUST be <language><?= $site->lang() ?></language>.
+<?php endif ?>
+</rules>
+
+<?php if (isset($instructions) && $instructions !== null && $instructions !== '') : ?>
+<user-instructions>
+	<?= $instructions ?>
+</user-instructions>
+<?php endif ?>
+
+<task>
+	Write descriptive alt text for the attached image.
+
+	The file is named <filename><?= $file->filename() ?></filename>.
+
+	<?php if ($file->parent() instanceof \Kirby\Cms\Page) : ?>
+	The image is on a page called <page-title><?= $file->parent()->title() ?></page-title>.
+	<?php endif ?>
+
+	<?php if ($file->template()) : ?>
+	This image uses the file template <template><?= $file->template() ?></template>.
+	<?php endif ?>
+
+<?php if (!empty($languages)) : ?>
+	Output alt text in these languages: <?= implode(', ', $languages) ?>
+
+<?php endif ?>
+</task>