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>

app/routes/platform_pages.py

@@ -158,18 +158,13 @@ async def homepage(
         return RedirectResponse(url="/shop/", status_code=302)
 
     # Scenario 2: Platform marketing site (no vendor)
-    # Try to load platform homepage from CMS
+    # Load platform homepage from CMS (slug='home')
     platform_id = platform.id if platform else 1
 
     cms_homepage = content_page_service.get_platform_page(
         db, platform_id=platform_id, slug="home", include_unpublished=False
     )
 
-    if not cms_homepage:
-        cms_homepage = content_page_service.get_platform_page(
-            db, platform_id=platform_id, slug="platform_homepage", include_unpublished=False
-        )
-
     if cms_homepage:
         # Use CMS-based homepage with template selection
         context = get_platform_context(request, db)