feat: improve Wagtail admin editor experience for articles

- Add published_date field to ArticlePage with auto-populate from
  first_published_at on first publish, plus data migration backfill
- Surface go_live_at/expire_at scheduling fields in editor panels
- Reorganise ArticlePage editor with TabbedInterface (Content,
  Metadata, Publishing, SEO tabs)
- Add Articles PageListingViewSet to admin menu with custom columns
  (author, category, published date, status) and category/author filters
- Add Articles summary dashboard panel showing drafts, scheduled,
  and recently published articles
- Update all front-end queries and RSS feeds to use published_date
- Add 10 unit tests and 4 E2E tests for new admin features

Closes #39

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/blog/tests/factories.py

@@ -37,6 +37,7 @@ class ArticlePageFactory(wagtail_factories.PageFactory):
     summary = "Summary"
     body = [("rich_text", "<p>Hello world</p>")]
     first_published_at = factory.LazyFunction(timezone.now)
+    published_date = factory.LazyFunction(timezone.now)
 
 
 class LegalIndexPageFactory(wagtail_factories.PageFactory):