feat: add platform detail/edit admin UI and service enhancements

- Add platform detail and edit admin pages with templates and JS
- Add ContentPageService methods: list_all_platform_pages, list_all_vendor_defaults
- Deprecate /admin/platform-homepage route (redirects to /admin/platforms)
- Add migration to fix content_page nullable columns
- Refine platform and vendor context middleware
- Add platform context middleware unit tests
- Update platforms.js with improved functionality
- Add section-based homepage plan documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/admin/js/platform-homepage.js

@@ -60,15 +60,15 @@ function platformHomepageManager() {
                 // Handle response - API returns array directly
                 const pages = Array.isArray(response) ? response : (response.data || response.items || []);
 
-                // Find the platform_homepage page
-                const homepage = pages.find(page => page.slug === 'platform_homepage');
+                // Find the homepage page (slug='home')
+                const homepage = pages.find(page => page.slug === 'home');
 
                 if (!homepage) {
                     platformHomepageLog.warn('Platform homepage not found, creating default...');
                     // Initialize with default values
                     this.page = {
                         id: null,
-                        slug: 'platform_homepage',
+                        slug: 'home',
                         title: 'Welcome to Our Multi-Vendor Marketplace',
                         content: '<p>Connect vendors with customers worldwide. Build your online store and reach millions of shoppers.</p>',
                         template: 'default',