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>

templates/blog/panels/articles_summary.html

@@ -0,0 +1,62 @@
+{% load wagtailadmin_tags %}
+<section class="nice-padding">
+    <h2 class="visuallyhidden">Articles overview</h2>
+
+    {% if drafts %}
+    <div class="w-mb-4">
+        <h3><svg class="icon icon-doc-empty" aria-hidden="true"><use href="#icon-doc-empty"></use></svg> Drafts</h3>
+        <table class="listing">
+            <tbody>
+                {% for page in drafts %}
+                <tr>
+                    <td class="title">
+                        <a href="{% url 'wagtailadmin_pages:edit' page.pk %}">{{ page.title }}</a>
+                    </td>
+                    <td>{{ page.latest_revision_created_at|timesince }} ago</td>
+                </tr>
+                {% endfor %}
+            </tbody>
+        </table>
+    </div>
+    {% endif %}
+
+    {% if scheduled %}
+    <div class="w-mb-4">
+        <h3><svg class="icon icon-time" aria-hidden="true"><use href="#icon-time"></use></svg> Scheduled</h3>
+        <table class="listing">
+            <tbody>
+                {% for page in scheduled %}
+                <tr>
+                    <td class="title">
+                        <a href="{% url 'wagtailadmin_pages:edit' page.pk %}">{{ page.title }}</a>
+                    </td>
+                    <td>{{ page.go_live_at|date:"N j, Y H:i" }}</td>
+                </tr>
+                {% endfor %}
+            </tbody>
+        </table>
+    </div>
+    {% endif %}
+
+    {% if recent %}
+    <div class="w-mb-4">
+        <h3><svg class="icon icon-doc-full" aria-hidden="true"><use href="#icon-doc-full"></use></svg> Recently published</h3>
+        <table class="listing">
+            <tbody>
+                {% for page in recent %}
+                <tr>
+                    <td class="title">
+                        <a href="{% url 'wagtailadmin_pages:edit' page.pk %}">{{ page.title }}</a>
+                    </td>
+                    <td>{{ page.published_date|timesince }} ago</td>
+                </tr>
+                {% endfor %}
+            </tbody>
+        </table>
+    </div>
+    {% endif %}
+
+    {% if not drafts and not scheduled and not recent %}
+    <p>No articles yet. <a href="{% url 'articles:choose_parent' %}">Create one</a>.</p>
+    {% endif %}
+</section>