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>

static/vendor/js/content-pages.js

@@ -24,6 +24,7 @@ function vendorContentPagesManager() {
         platformPages: [],  // Platform default pages
         customPages: [],    // Vendor's own pages (overrides + custom)
         overrideMap: {},    // Map of slug -> page id for quick lookup
+        cmsUsage: null,     // CMS usage statistics
 
         // Initialize
         async init() {
@@ -43,7 +44,10 @@ function vendorContentPagesManager() {
                     await parentInit.call(this);
                 }
 
-                await this.loadPages();
+                await Promise.all([
+                    this.loadPages(),
+                    this.loadCmsUsage()
+                ]);
 
                 contentPagesLog.info('=== VENDOR CONTENT PAGES MANAGER INITIALIZATION COMPLETE ===');
             } catch (error) {
@@ -92,6 +96,19 @@ function vendorContentPagesManager() {
             }
         },
 
+        // Load CMS usage statistics
+        async loadCmsUsage() {
+            try {
+                contentPagesLog.info('Loading CMS usage...');
+                const response = await apiClient.get('/vendor/content-pages/usage');
+                this.cmsUsage = response.data || response;
+                contentPagesLog.info('CMS usage loaded:', this.cmsUsage);
+            } catch (err) {
+                contentPagesLog.error('Error loading CMS usage:', err);
+                // Non-critical - don't set error state
+            }
+        },
+
         // Check if vendor has overridden a platform page
         hasOverride(slug) {
             return slug in this.overrideMap;