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/api/v1/admin/__init__.py

@@ -47,6 +47,7 @@ from . import (
     order_item_exceptions,
     orders,
     platform_health,
+    platforms,
     products,
     settings,
     subscriptions,
@@ -91,6 +92,9 @@ router.include_router(
     content_pages.router, prefix="/content-pages", tags=["admin-content-pages"]
 )
 
+# Include platforms management endpoints (multi-platform CMS)
+router.include_router(platforms.router, tags=["admin-platforms"])
+
 
 # ============================================================================
 # User Management