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/migrations/0004_backfill_published_date.py

@@ -0,0 +1,24 @@
+# Generated by Django 5.2.11 on 2026-03-03 13:59
+
+from django.db import migrations
+
+
+def backfill_published_date(apps, schema_editor):
+    schema_editor.execute(
+        "UPDATE blog_articlepage SET published_date = p.first_published_at "
+        "FROM wagtailcore_page p "
+        "WHERE blog_articlepage.page_ptr_id = p.id "
+        "AND blog_articlepage.published_date IS NULL "
+        "AND p.first_published_at IS NOT NULL"
+    )
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('blog', '0003_add_published_date'),
+    ]
+
+    operations = [
+        migrations.RunPython(backfill_published_date, migrations.RunPython.noop),
+    ]