feat: add CMS database model and migrations

Implement Content Management System database layer:

Database Model:
- ContentPage model with two-tier architecture
- Platform defaults (vendor_id=NULL)
- Vendor-specific overrides (vendor_id=123)
- SEO fields (meta_description, meta_keywords)
- Publishing workflow (is_published, published_at)
- Navigation flags (show_in_footer, show_in_header)
- Display ordering and timestamps

Migrations:
- Create content_pages table with all columns
- Add indexes for performance (vendor_id, slug, published status)
- Add unique constraint on (vendor_id, slug)
- Add foreign key relationships with cascade delete

Model Registration:
- Add ContentPage to Vendor relationship
- Import model in alembic/env.py for migration detection

This provides the foundation for managing static content pages
(About, FAQ, Contact, etc.) with platform defaults and vendor overrides.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

alembic/env.py

@@ -93,6 +93,16 @@ try:
 except ImportError as e:
     print(f"  ✗ VendorTheme model failed: {e}")
 
+# ----------------------------------------------------------------------------
+# CONTENT PAGE MODEL (CMS)
+# ----------------------------------------------------------------------------
+try:
+    from models.database.content_page import ContentPage
+
+    print("  ✓ ContentPage model imported")
+except ImportError as e:
+    print(f"  ✗ ContentPage model failed: {e}")
+
 # ----------------------------------------------------------------------------
 # PRODUCT MODELS
 # ----------------------------------------------------------------------------