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/routes/shop_pages.py

@@ -114,10 +114,14 @@ def get_shop_context(request: Request, db: Session = None, **extra_context) -> d
     """
     # Extract from middleware state
     vendor = getattr(request.state, "vendor", None)
+    platform = getattr(request.state, "platform", None)
     theme = getattr(request.state, "theme", None)
     clean_path = getattr(request.state, "clean_path", request.url.path)
     vendor_context = getattr(request.state, "vendor_context", None)
 
+    # Get platform_id (default to 1 for OMS if not set)
+    platform_id = platform.id if platform else 1
+
     # Get detection method from vendor_context
     access_method = (
         vendor_context.get("detection_method", "unknown")
@@ -156,11 +160,11 @@ def get_shop_context(request: Request, db: Session = None, **extra_context) -> d
             vendor_id = vendor.id
             # Get pages configured to show in footer
             footer_pages = content_page_service.list_pages_for_vendor(
-                db, vendor_id=vendor_id, footer_only=True, include_unpublished=False
+                db, platform_id=platform_id, vendor_id=vendor_id, footer_only=True, include_unpublished=False
             )
             # Get pages configured to show in header
             header_pages = content_page_service.list_pages_for_vendor(
-                db, vendor_id=vendor_id, header_only=True, include_unpublished=False
+                db, platform_id=platform_id, vendor_id=vendor_id, header_only=True, include_unpublished=False
             )
         except Exception as e:
             logger.error(
@@ -752,11 +756,13 @@ async def generic_content_page(
     )
 
     vendor = getattr(request.state, "vendor", None)
+    platform = getattr(request.state, "platform", None)
     vendor_id = vendor.id if vendor else None
+    platform_id = platform.id if platform else 1  # Default to OMS
 
-    # Load content page from database (vendor override → platform default)
+    # Load content page from database (vendor override → vendor default)
     page = content_page_service.get_page_for_vendor(
-        db, slug=slug, vendor_id=vendor_id, include_unpublished=False
+        db, platform_id=platform_id, slug=slug, vendor_id=vendor_id, include_unpublished=False
     )
 
     if not page: