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/migrations/versions/cms_003_seo_meta_description_translations.py

@@ -0,0 +1,36 @@
+"""add meta_description_translations and drop meta_keywords from content_pages
+
+Revision ID: cms_003
+Revises: cms_002
+Create Date: 2026-04-15
+"""
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "cms_003"
+down_revision = "cms_002"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "content_pages",
+        sa.Column(
+            "meta_description_translations",
+            sa.JSON(),
+            nullable=True,
+            comment="Language-keyed meta description dict for multi-language SEO",
+        ),
+    )
+    op.drop_column("content_pages", "meta_keywords")
+
+
+def downgrade() -> None:
+    op.add_column(
+        "content_pages",
+        sa.Column("meta_keywords", sa.String(300), nullable=True),
+    )
+    op.drop_column("content_pages", "meta_description_translations")