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>

e2e/test_admin_experience.py

@@ -0,0 +1,56 @@
+"""E2E tests for Wagtail admin editor experience improvements."""
+
+from __future__ import annotations
+
+import pytest
+from playwright.sync_api import Page, expect
+
+
+def admin_login(page: Page, base_url: str) -> None:
+    """Log in to the Wagtail admin using the seeded E2E admin user."""
+    page.goto(f"{base_url}/cms/login/", wait_until="networkidle")
+    page.fill('input[name="username"]', "e2e-admin")
+    page.fill('input[name="password"]', "e2e-admin-pass")
+    page.click('button[type="submit"]')
+    page.wait_for_load_state("networkidle")
+
+
+@pytest.mark.e2e
+def test_articles_menu_item_visible(page: Page, base_url: str) -> None:
+    """The admin sidebar should contain an 'Articles' menu item."""
+    admin_login(page, base_url)
+    sidebar = page.locator("[data-side-panel]").first
+    articles_link = sidebar.get_by_role("link", name="Articles")
+    expect(articles_link).to_be_visible()
+
+
+@pytest.mark.e2e
+def test_articles_listing_page_loads(page: Page, base_url: str) -> None:
+    """Clicking 'Articles' should load the articles listing with seeded articles."""
+    admin_login(page, base_url)
+    page.goto(f"{base_url}/cms/articles/", wait_until="networkidle")
+    expect(page.get_by_role("heading").first).to_be_visible()
+    # Seeded articles should appear
+    expect(page.get_by_text("Nightly Playwright Journey")).to_be_visible()
+
+
+@pytest.mark.e2e
+def test_dashboard_has_articles_panel(page: Page, base_url: str) -> None:
+    """The admin dashboard should include the articles summary panel."""
+    admin_login(page, base_url)
+    page.goto(f"{base_url}/cms/", wait_until="networkidle")
+    expect(page.get_by_text("Articles overview")).to_be_visible()
+
+
+@pytest.mark.e2e
+def test_article_editor_has_tabs(page: Page, base_url: str) -> None:
+    """The article editor should have Content, Metadata, Publishing, and SEO tabs."""
+    admin_login(page, base_url)
+    page.goto(f"{base_url}/cms/articles/", wait_until="networkidle")
+    # Click the first article to edit it
+    page.get_by_text("Nightly Playwright Journey").first.click()
+    page.wait_for_load_state("networkidle")
+    expect(page.get_by_role("tab", name="Content")).to_be_visible()
+    expect(page.get_by_role("tab", name="Metadata")).to_be_visible()
+    expect(page.get_by_role("tab", name="Publishing")).to_be_visible()
+    expect(page.get_by_role("tab", name="SEO")).to_be_visible()