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/models.py

@@ -11,7 +11,7 @@ from django.shortcuts import get_object_or_404
 from modelcluster.contrib.taggit import ClusterTaggableManager
 from modelcluster.fields import ParentalKey
 from taggit.models import Tag, TaggedItemBase
-from wagtail.admin.panels import FieldPanel, PageChooserPanel
+from wagtail.admin.panels import FieldPanel, ObjectList, PageChooserPanel, TabbedInterface
 from wagtail.contrib.routable_page.models import RoutablePageMixin, route
 from wagtail.fields import RichTextField, StreamField
 from wagtail.models import Page
@@ -38,7 +38,7 @@ class HomePage(Page):
             .public()
             .select_related("author", "category")
             .prefetch_related("tags__metadata")
-            .order_by("-first_published_at")
+            .order_by("-published_date")
         )
         articles = list(articles_qs[:5])
         ctx["featured_article"] = self.featured_article
@@ -64,7 +64,7 @@ class ArticleIndexPage(RoutablePageMixin, Page):
             .live()
             .select_related("author", "category")
             .prefetch_related("tags__metadata")
-            .order_by("-first_published_at")
+            .order_by("-published_date")
         )
 
     def get_category_url(self, category):
@@ -191,21 +191,46 @@ class ArticlePage(SeoMixin, Page):
     tags = ClusterTaggableManager(through="blog.ArticleTag", blank=True)
     read_time_mins = models.PositiveIntegerField(editable=False, default=1)
     comments_enabled = models.BooleanField(default=True)
+    published_date = models.DateTimeField(
+        null=True,
+        blank=True,
+        help_text="Display date for this article. Auto-set on first publish if left blank.",
+    )
 
     parent_page_types = ["blog.ArticleIndexPage"]
     subpage_types: list[str] = []
 
-    content_panels = Page.content_panels + [
-        FieldPanel("category"),
-        FieldPanel("author"),
-        FieldPanel("hero_image"),
+    content_panels = [
+        FieldPanel("title"),
         FieldPanel("summary"),
         FieldPanel("body"),
+    ]
+
+    metadata_panels = [
+        FieldPanel("category"),
+        FieldPanel("author"),
         FieldPanel("tags"),
+        FieldPanel("hero_image"),
         FieldPanel("comments_enabled"),
     ]
 
-    promote_panels = Page.promote_panels + SeoMixin.seo_panels
+    publishing_panels = [
+        FieldPanel("published_date"),
+        FieldPanel("go_live_at"),
+        FieldPanel("expire_at"),
+    ]
+
+    edit_handler = TabbedInterface(
+        [
+            ObjectList(content_panels, heading="Content"),
+            ObjectList(metadata_panels, heading="Metadata"),
+            ObjectList(publishing_panels, heading="Publishing"),
+            ObjectList(
+                Page.promote_panels + SeoMixin.seo_panels,
+                heading="SEO",
+            ),
+        ]
+    )
 
     search_fields = Page.search_fields
 
@@ -215,6 +240,8 @@ class ArticlePage(SeoMixin, Page):
                 slug="general",
                 defaults={"name": "General", "description": "General articles", "colour": "neutral"},
             )
+        if not self.published_date and self.first_published_at:
+            self.published_date = self.first_published_at
         self.read_time_mins = self._compute_read_time()
         return super().save(*args, **kwargs)
 
@@ -239,14 +266,14 @@ class ArticlePage(SeoMixin, Page):
             .filter(tags__in=tag_ids)
             .exclude(pk=self.pk)
             .distinct()
-            .order_by("-first_published_at")[:count]
+            .order_by("-published_date")[:count]
         )
         if len(related) < count:
             exclude_ids = [a.pk for a in related] + [self.pk]
             fallback = list(
                 ArticlePage.objects.live()
                 .exclude(pk__in=exclude_ids)
-                .order_by("-first_published_at")[: count - len(related)]
+                .order_by("-published_date")[: count - len(related)]
             )
             return related + fallback
         return related