feat: complete multi-platform CMS phases 2-5

Phase 2 - OMS Migration & Integration:
- Fix platform_pages.py to use get_platform_page for marketing pages
- Fix shop_pages.py to pass platform_id to content page service calls

Phase 3 - Admin Interface:
- Add platform management API (app/api/v1/admin/platforms.py)
- Add platforms admin page with stats cards
- Add Platforms menu item to admin sidebar
- Update content pages admin with platform filter and four-tab tier system

Phase 4 - Documentation:
- Add comprehensive architecture docs (docs/architecture/multi-platform-cms.md)
- Update implementation plan with completion status

Phase 5 - Vendor Dashboard:
- Add CMS usage API endpoint with tier limits
- Add usage progress bar to vendor content pages
- Add platform-default/{slug} API for preview
- Add View Default button and modal in page editor

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/database/content_page.py

@@ -191,6 +191,7 @@ class ContentPage(Base):
             "id": self.id,
             "platform_id": self.platform_id,
             "platform_code": self.platform.code if self.platform else None,
+            "platform_name": self.platform.name if self.platform else None,
             "vendor_id": self.vendor_id,
             "vendor_name": self.vendor.name if self.vendor else None,
             "slug": self.slug,