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/classes/AltText.php

@@ -0,0 +1,126 @@
+<?php
+
+namespace tobimori\Seo;
+
+use Kirby\Content\Field;
+use Kirby\Data\Yaml;
+
+/**
+ * Value object for structured alt text data.
+ * Handles both YAML object format and plain string migration.
+ */
+class AltText
+{
+	public const string SOURCE_AI = 'ai';
+	public const string SOURCE_MANUAL = 'manual';
+	public const string SOURCE_REVIEWED = 'reviewed';
+
+	public function __construct(
+		protected string $text = '',
+		protected bool $decorative = false,
+		protected string $source = self::SOURCE_MANUAL,
+	) {
+	}
+
+	/**
+	 * Parses a raw field value into an AltText instance.
+	 * Handles YAML object format and plain string migration.
+	 */
+	public static function parse(string|null $value): static
+	{
+		if ($value === null || $value === '') {
+			return new static();
+		}
+
+		try {
+			$data = Yaml::decode($value);
+
+			if (is_array($data) && (array_key_exists('text', $data) || array_key_exists('decorative', $data))) {
+				return new static(
+					text: (string)($data['text'] ?? ''),
+					decorative: (bool)($data['decorative'] ?? false),
+					source: (string)($data['source'] ?? self::SOURCE_MANUAL),
+				);
+			}
+		} catch (\Throwable) {
+			// not valid YAML
+		}
+
+		// plain string = migration from old alt field
+		return new static(text: $value, source: self::SOURCE_MANUAL);
+	}
+
+	public static function fromField(Field $field): static
+	{
+		return static::parse($field->value());
+	}
+
+	public function text(): string
+	{
+		return $this->text;
+	}
+
+	public function isDecorative(): bool
+	{
+		return $this->decorative;
+	}
+
+	public function source(): string
+	{
+		return $this->source;
+	}
+
+	public function isMissing(): bool
+	{
+		return !$this->decorative && trim($this->text) === '';
+	}
+
+	public function isAiGenerated(): bool
+	{
+		return $this->source === self::SOURCE_AI;
+	}
+
+	public function isReviewed(): bool
+	{
+		return $this->source === self::SOURCE_REVIEWED;
+	}
+
+	public function toArray(): array
+	{
+		return [
+			'text' => $this->text,
+			'decorative' => $this->decorative,
+			'source' => $this->source,
+		];
+	}
+
+	/**
+	 * Returns HTML attributes for the alt text.
+	 */
+	public function toAttr(): array
+	{
+		return ['alt' => $this->decorative ? '' : $this->text];
+	}
+
+	public function toYaml(): string
+	{
+		return Yaml::encode($this->toArray());
+	}
+
+	/**
+	 * Returns the resolved alt text as a Kirby Field for chaining (.or(), .isNotEmpty(), etc.)
+	 */
+	public function toField(): Field
+	{
+		return new Field(null, 'alt', (string)$this);
+	}
+
+	public function __toString(): string
+	{
+		if ($this->decorative) {
+			return '';
+		}
+
+		return $this->text;
+	}
+}