feat: add custom schema, AI generation, XML sitemap, platform images

- Custom JSON-LD schema builder: per-article textarea for arbitrary
  structured data with JSON validation. Closes #70
- AI-powered meta generation: Generate with AI buttons for OG title
  and description, supports Claude and OpenAI APIs. Closes #71
- XML sitemap: auto-generates sitemap.xml on article save, respects
  noindex directives. Closes #72
- Per-platform image resizing: Twitter 1200x600, Pinterest 1000x1500,
  WhatsApp 400x400 alongside default Facebook 1200x630. Closes #74
- DB migration 01.05.00: adds custom_schema column

source/packages/com_mokoog/sql/install.mysql.sql

@@ -15,6 +15,7 @@ CREATE TABLE IF NOT EXISTS `#__mokoog_tags` (
     `og_video`       VARCHAR(512) NOT NULL DEFAULT '',
     `event_data`     TEXT NULL,
     `recipe_data`    TEXT NULL,
+    `custom_schema`  TEXT NULL,
     `seo_title`      VARCHAR(70) NOT NULL DEFAULT '',
     `meta_description` VARCHAR(200) NOT NULL DEFAULT '',
     `robots`         VARCHAR(100) NOT NULL DEFAULT '',