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-page-edit.js

@@ -35,6 +35,11 @@ function vendorContentPageEditor(pageId) {
         error: null,
         successMessage: null,
 
+        // Default preview modal state
+        showingDefaultPreview: false,
+        loadingDefault: false,
+        defaultContent: null,
+
         // Initialize
         async init() {
             // Prevent multiple initializations
@@ -179,6 +184,31 @@ function vendorContentPageEditor(pageId) {
             }
         },
 
+        // Show default content preview
+        async showDefaultPreview() {
+            this.showingDefaultPreview = true;
+            this.loadingDefault = true;
+            this.defaultContent = null;
+
+            try {
+                contentPageEditLog.info('Loading platform default for slug:', this.form.slug);
+
+                const response = await apiClient.get(`/vendor/content-pages/platform-default/${this.form.slug}`);
+                this.defaultContent = response.data || response;
+
+                contentPageEditLog.info('Default content loaded');
+
+            } catch (err) {
+                contentPageEditLog.error('Error loading default content:', err);
+                this.defaultContent = {
+                    title: 'Error',
+                    content: `<p class="text-red-500">Failed to load platform default: ${err.message}</p>`
+                };
+            } finally {
+                this.loadingDefault = false;
+            }
+        },
+
         // Delete page (revert to default for overrides)
         async deletePage() {
             const message = this.isOverride