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/models/content_page.py

@@ -135,7 +135,12 @@ class ContentPage(Base):
 
     # SEO
     meta_description = Column(String(300), nullable=True)
-    meta_keywords = Column(String(300), nullable=True)
+    meta_description_translations = Column(
+        JSON,
+        nullable=True,
+        default=None,
+        comment="Language-keyed meta description dict for multi-language SEO",
+    )
 
     # Publishing
     is_published = Column(Boolean, default=False, nullable=False)
@@ -230,6 +235,16 @@ class ContentPage(Base):
             )
         return self.content
 
+    def get_translated_meta_description(self, lang: str, default_lang: str = "fr") -> str:
+        """Get meta description in the given language, falling back to default_lang then self.meta_description."""
+        if self.meta_description_translations:
+            return (
+                self.meta_description_translations.get(lang)
+                or self.meta_description_translations.get(default_lang)
+                or self.meta_description or ""
+            )
+        return self.meta_description or ""
+
     def to_dict(self):
         """Convert to dictionary for API responses."""
         return {
@@ -248,7 +263,7 @@ class ContentPage(Base):
             "template": self.template,
             "sections": self.sections,
             "meta_description": self.meta_description,
-            "meta_keywords": self.meta_keywords,
+            "meta_description_translations": self.meta_description_translations,
             "is_published": self.is_published,
             "published_at": (
                 self.published_at.isoformat() if self.published_at else None