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>

models/database/vendor.py

@@ -80,6 +80,13 @@ class Vendor(Base, TimestampMixin):
         cascade="all, delete-orphan"
     )  # Relationship with VendorTheme model for the active theme of the vendor
 
+    # Content pages relationship (vendor can override platform default pages)
+    content_pages = relationship(
+        "ContentPage",
+        back_populates="vendor",
+        cascade="all, delete-orphan"
+    )  # Relationship with ContentPage model for vendor-specific content pages
+
     def __repr__(self):
         """String representation of the Vendor object."""
         return f"<Vendor(id={self.id}, vendor_code='{self.vendor_code}', name='{self.name}', subdomain='{self.subdomain}')>"