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/modules/cms/routes/api/admin_content_pages.py

@@ -73,7 +73,7 @@ def create_platform_page(
         content_format=page_data.content_format,
         template=page_data.template,
         meta_description=page_data.meta_description,
-        meta_keywords=page_data.meta_keywords,
+        meta_description_translations=page_data.meta_description_translations,
         is_published=page_data.is_published,
         show_in_footer=page_data.show_in_footer,
         show_in_header=page_data.show_in_header,
@@ -117,7 +117,7 @@ def create_store_page(
         content_format=page_data.content_format,
         template=page_data.template,
         meta_description=page_data.meta_description,
-        meta_keywords=page_data.meta_keywords,
+        meta_description_translations=page_data.meta_description_translations,
         is_published=page_data.is_published,
         show_in_footer=page_data.show_in_footer,
         show_in_header=page_data.show_in_header,
@@ -177,11 +177,13 @@ def update_page(
         db,
         page_id=page_id,
         title=page_data.title,
+        title_translations=page_data.title_translations,
         content=page_data.content,
+        content_translations=page_data.content_translations,
         content_format=page_data.content_format,
         template=page_data.template,
         meta_description=page_data.meta_description,
-        meta_keywords=page_data.meta_keywords,
+        meta_description_translations=page_data.meta_description_translations,
         is_published=page_data.is_published,
         show_in_footer=page_data.show_in_footer,
         show_in_header=page_data.show_in_header,