feat(cms): Phase A — page type selector, translation UI, SEO cleanup

Content page editor improvements:
- Page type selector: Content Page / Landing Page dropdown (sets template)
- Title language tabs: translate page titles per language (same pattern as sections)
- Content language tabs: translate page content per language
- Meta description language tabs: translatable SEO descriptions
- Template-driven section palette: template defines which sections are available
  (store landing pages hide Pricing, platform homepages show all)
- Hide content editor when Landing Page selected, hide sections when Content Page

Schema changes (migration cms_003):
- Add meta_description_translations column (JSON) to content_pages
- Drop meta_keywords column (obsolete, ignored by all search engines since 2009)
- Remove meta keywords tag from storefront and platform base templates

API + service updates:
- title_translations, content_translations, meta_description_translations
  added to create/update schemas, route handlers, and service methods

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/templates/storefront/base.html

@@ -14,7 +14,6 @@
 
     {# SEO Meta Tags #}
     <meta name="description" content="{% block meta_description %}{{ store.description or 'Shop at ' + store.name }}{% endblock %}">
-    <meta name="keywords" content="{% block meta_keywords %}{{ store.name }}, online shop{% endblock %}">
 
     {# Favicon - store-specific or default #}
     {% if theme.branding.favicon %}