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/core/management/commands/seed_e2e_content.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from django.contrib.auth import get_user_model
 from django.core.management.base import BaseCommand
 from taggit.models import Tag
 from wagtail.models import Page, Site
@@ -10,6 +11,8 @@ from apps.comments.models import Comment
 from apps.core.models import NavigationMenuItem, SiteSettings, SocialMediaLink
 from apps.legal.models import LegalIndexPage, LegalPage
 
+User = get_user_model()
+
 
 class Command(BaseCommand):
     help = "Seed deterministic content for E2E checks."
@@ -181,4 +184,12 @@ class Command(BaseCommand):
             ]
         )
 
+        # Admin user for E2E admin tests
+        if not User.objects.filter(username="e2e-admin").exists():
+            User.objects.create_superuser(
+                username="e2e-admin",
+                email="e2e-admin@example.com",
+                password="e2e-admin-pass",
+            )
+
         self.stdout.write(self.style.SUCCESS("Seeded E2E content."))