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>

app/templates/admin/partials/sidebar.html

@@ -104,6 +104,7 @@
     <!-- Content Management Section -->
     {{ section_header('Content Management', 'contentMgmt') }}
     {% call section_content('contentMgmt') %}
+        {{ menu_item('platforms', '/admin/platforms', 'globe-alt', 'Platforms') }}
         {{ menu_item('platform-homepage', '/admin/platform-homepage', 'home', 'Platform Homepage') }}
         {{ menu_item('content-pages', '/admin/content-pages', 'document-text', 'Content Pages') }}
         {{ menu_item('vendor-theme', '/admin/vendor-themes', 'color-swatch', 'Vendor Themes') }}