refactor(admin): Migrate AdminAjaxHandler to Clean Architecture

- Move AdminAjaxHandler to Admin/Shared/Infrastructure/Api/Wordpress/
- Create FieldMapperInterface for decentralized field mapping
- Create FieldMapperRegistry for module discovery
- Create FieldMapperProvider for auto-registration of 12 mappers
- Add FieldMappers for all components:
  - ContactFormFieldMapper (46 fields)
  - CtaBoxSidebarFieldMapper (32 fields)
  - CtaLetsTalkFieldMapper
  - CtaPostFieldMapper
  - FeaturedImageFieldMapper (15 fields)
  - FooterFieldMapper (31 fields)
  - HeroFieldMapper
  - NavbarFieldMapper
  - RelatedPostFieldMapper (34 fields)
  - SocialShareFieldMapper
  - TableOfContentsFieldMapper
  - TopNotificationBarFieldMapper (17 fields)
- Update functions.php bootstrap with FieldMapperProvider
- AdminAjaxHandler reduced from ~700 to 145 lines
- Follows SRP, OCP, DIP principles

BACKUP BEFORE: Removing CTA A/B Testing legacy system

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Admin/Shared/Domain/Contracts/FieldMapperInterface.php

@@ -0,0 +1,38 @@
+<?php
+declare(strict_types=1);
+
+namespace ROITheme\Admin\Shared\Domain\Contracts;
+
+/**
+ * Contrato para mapeo de campos de formulario a atributos de BD
+ *
+ * RESPONSABILIDAD:
+ * - Definir el mapeo de field IDs a grupos/atributos
+ * - Cada modulo implementa su propio mapper
+ *
+ * PRINCIPIOS:
+ * - ISP: Interfaz pequena (2 metodos)
+ * - DIP: Capas superiores dependen de esta abstraccion
+ */
+interface FieldMapperInterface
+{
+    /**
+     * Retorna el nombre del componente que mapea
+     *
+     * @return string Nombre en kebab-case (ej: 'cta-box-sidebar')
+     */
+    public function getComponentName(): string;
+
+    /**
+     * Retorna el mapeo de field IDs a grupo/atributo
+     *
+     * @return array<string, array{group: string, attribute: string}>
+     *
+     * Ejemplo:
+     * [
+     *     'ctaTitle' => ['group' => 'content', 'attribute' => 'title'],
+     *     'ctaEnabled' => ['group' => 'visibility', 'attribute' => 'is_enabled'],
+     * ]
+     */
+    public function getFieldMapping(): array;
+}