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/0003_add_published_date.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.11 on 2026-03-03 13:59
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('blog', '0002_category_articlepage_category'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='articlepage',
+            name='published_date',
+            field=models.DateTimeField(blank=True, help_text='Display date for this article. Auto-set on first publish if left blank.', null=True),
+        ),
+    ]