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/admin_pages.py

@@ -1116,23 +1116,21 @@ async def admin_platform_edit(
 # ============================================================================
 
 
-@router.get("/platform-homepage", response_class=HTMLResponse, include_in_schema=False)
+@router.get("/platform-homepage", include_in_schema=False)
 async def admin_platform_homepage_manager(
     request: Request,
     current_user: User = Depends(get_current_admin_from_cookie_or_header),
     db: Session = Depends(get_db),
 ):
     """
-    Render platform homepage manager.
-    Allows editing the main platform homepage with template selection.
+    Deprecated: Redirects to platforms page.
+
+    Platform homepages are now managed via:
+    - /admin/platforms → Select platform → Homepage button
+    - Or directly: /admin/content-pages?platform_code={code}&slug=home
     """
-    return templates.TemplateResponse(
-        "admin/platform-homepage.html",
-        {
-            "request": request,
-            "user": current_user,
-        },
-    )
+    from starlette.responses import RedirectResponse
+    return RedirectResponse(url="/admin/platforms", status_code=302)
 
 
 @router.get("/content-pages", response_class=HTMLResponse, include_in_schema=False)