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/content_pages.py

@@ -82,6 +82,9 @@ class ContentPageResponse(BaseModel):
     """Schema for content page response."""
 
     id: int
+    platform_id: int | None = None
+    platform_code: str | None = None
+    platform_name: str | None = None
     vendor_id: int | None
     vendor_name: str | None
     slug: str
@@ -97,8 +100,11 @@ class ContentPageResponse(BaseModel):
     show_in_footer: bool
     show_in_header: bool
     show_in_legal: bool
-    is_platform_default: bool
-    is_vendor_override: bool
+    is_platform_page: bool = False
+    is_platform_default: bool = False  # Deprecated: use is_platform_page
+    is_vendor_default: bool = False
+    is_vendor_override: bool = False
+    page_tier: str | None = None
     created_at: str
     updated_at: str
     created_by: int | None