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/platforms.js

@@ -8,6 +8,12 @@ const platformsLog = window.LogConfig.createLogger('PLATFORMS');
 
 function platformsManager() {
   return {
+    // Inherit base layout functionality from init-alpine.js
+    ...data(),
+
+    // Page identification
+    currentPage: "platforms",
+
     // State
     platforms: [],
     loading: true,
@@ -15,12 +21,22 @@ function platformsManager() {
 
     // Lifecycle
     async init() {
+      platformsLog.info('=== PLATFORMS PAGE INITIALIZING ===');
+
       // Duplicate initialization guard
-      if (window._adminPlatformsInitialized) return;
+      if (window._adminPlatformsInitialized) {
+        platformsLog.warn('Platforms page already initialized, skipping...');
+        return;
+      }
       window._adminPlatformsInitialized = true;
 
-      this.currentPage = "platforms";
-      await this.loadPlatforms();
+      try {
+        await this.loadPlatforms();
+        platformsLog.info('=== PLATFORMS PAGE INITIALIZATION COMPLETE ===');
+      } catch (error) {
+        window.LogConfig.logError(error, 'Platforms Init');
+        this.error = 'Failed to initialize page';
+      }
     },
 
     // API Methods