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/content-pages.js

@@ -44,13 +44,19 @@ function contentPagesManager() {
             ]);
             contentPagesLog.groupEnd();
 
-            // Check for platform filter in URL
+            // Check for platform filter in URL (support both 'platform' and 'platform_code')
             const urlParams = new URLSearchParams(window.location.search);
-            const platformParam = urlParams.get('platform');
+            const platformParam = urlParams.get('platform_code') || urlParams.get('platform');
             if (platformParam) {
                 this.selectedPlatform = platformParam;
             }
 
+            // Check for slug param - if specified, redirect to edit page
+            const slugParam = urlParams.get('slug');
+            if (slugParam && platformParam) {
+                await this.redirectToEditIfSlugMatches(platformParam, slugParam);
+            }
+
             contentPagesLog.info('=== CONTENT PAGES MANAGER INITIALIZATION COMPLETE ===');
         },
 
@@ -162,6 +168,30 @@ function contentPagesManager() {
             }
         },
 
+        // Redirect to edit page if a specific slug is requested
+        async redirectToEditIfSlugMatches(platformCode, slug) {
+            contentPagesLog.info(`Looking for page with platform=${platformCode}, slug=${slug}`);
+
+            // Find the page matching the platform and slug
+            const matchingPage = this.allPages.find(page =>
+                page.platform_code === platformCode && page.slug === slug
+            );
+
+            if (matchingPage) {
+                contentPagesLog.info(`Found matching page: ${matchingPage.id}, redirecting to edit...`);
+                window.location.href = `/admin/content-pages/${matchingPage.id}/edit`;
+            } else {
+                contentPagesLog.warn(`No page found for platform=${platformCode}, slug=${slug}`);
+                // Show a toast and offer to create
+                if (slug === 'home') {
+                    // Offer to create homepage
+                    if (confirm(`No homepage found for ${platformCode}. Would you like to create one?`)) {
+                        window.location.href = `/admin/content-pages/create?platform_code=${platformCode}&slug=home&is_platform_page=true`;
+                    }
+                }
+            }
+        },
+
         // Get page tier label (three-tier system)
         getPageTierLabel(page) {
             if (page.vendor_id) {