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/feeds.py

@@ -16,7 +16,7 @@ class AllArticlesFeed(Feed):
         return None
 
     def items(self):
-        return ArticlePage.objects.live().order_by("-first_published_at")[:20]
+        return ArticlePage.objects.live().order_by("-published_date")[:20]
 
     def item_title(self, item: ArticlePage):
         return item.title
@@ -25,7 +25,7 @@ class AllArticlesFeed(Feed):
         return item.summary
 
     def item_pubdate(self, item: ArticlePage):
-        return item.first_published_at
+        return item.published_date or item.first_published_at
 
     def item_author_name(self, item: ArticlePage):
         return item.author.name
@@ -47,7 +47,7 @@ class TagArticlesFeed(AllArticlesFeed):
         return f"No Hype AI — {obj.name}"
 
     def items(self, obj):
-        return ArticlePage.objects.live().filter(tags=obj).order_by("-first_published_at")[:20]
+        return ArticlePage.objects.live().filter(tags=obj).order_by("-published_date")[:20]
 
 
 class CategoryArticlesFeed(AllArticlesFeed):
@@ -59,4 +59,4 @@ class CategoryArticlesFeed(AllArticlesFeed):
         return f"No Hype AI — {obj.name}"
 
     def items(self, obj):
-        return ArticlePage.objects.live().filter(category=obj).order_by("-first_published_at")[:20]
+        return ArticlePage.objects.live().filter(category=obj).order_by("-published_date")[:20]