fix: resolve ruff/mypy lint errors and fix E2E test failures

- Use datetime.timedelta instead of timezone.timedelta (mypy)
- Fix import ordering (ruff I001)
- Fix admin sidebar E2E selector: use #wagtail-sidebar (Wagtail 7)
- Set deterministic published_date on seeded E2E articles for stable ordering
- Fix nightly test strict-mode violation: exact=True for Comments heading

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/core/management/commands/seed_e2e_content.py

@@ -20,6 +20,8 @@ class Command(BaseCommand):
     def handle(self, *args, **options):
         import datetime
 
+        from django.utils import timezone
+
         root = Page.get_first_root_node()
 
         home = HomePage.objects.child_of(root).first()
@@ -43,6 +45,9 @@ class Command(BaseCommand):
         )
 
         # Primary article — comments enabled, used by nightly journey test
+        # published_date is set explicitly to ensure deterministic ordering
+        # (most recent first) so this article appears at the top of listings.
+        now = timezone.now()
         article = ArticlePage.objects.child_of(article_index).filter(slug="nightly-playwright-journey").first()
         if article is None:
             article = ArticlePage(
@@ -52,9 +57,12 @@ class Command(BaseCommand):
                 summary="Seeded article for nightly browser journey.",
                 body=[("rich_text", "<p>Seeded article body for nightly browser checks.</p>")],
                 comments_enabled=True,
+                published_date=now,
             )
             article_index.add_child(instance=article)
             article.save_revision().publish()
+        # Ensure deterministic ordering — primary article always newest
+        ArticlePage.objects.filter(pk=article.pk).update(published_date=now)
 
         # Seed one approved top-level comment on the primary article for reply E2E tests
         if not Comment.objects.filter(article=article, author_name="E2E Approved Commenter").exists():
@@ -78,9 +86,13 @@ class Command(BaseCommand):
                 summary="An article with tags for E2E filter tests.",
                 body=[("rich_text", "<p>This article is tagged with AI Tools.</p>")],
                 comments_enabled=True,
+                published_date=now - datetime.timedelta(hours=1),
             )
             article_index.add_child(instance=tagged_article)
             tagged_article.save_revision().publish()
+        ArticlePage.objects.filter(pk=tagged_article.pk).update(
+            published_date=now - datetime.timedelta(hours=1)
+        )
         tagged_article.tags.add(tag)
         tagged_article.save()
 
@@ -94,6 +106,7 @@ class Command(BaseCommand):
                 summary="An article with comments disabled.",
                 body=[("rich_text", "<p>Comments are disabled on this one.</p>")],
                 comments_enabled=False,
+                published_date=now - datetime.timedelta(hours=2),
             )
             article_index.add_child(instance=no_comments_article)
             # Explicitly persist False after add_child (which internally calls save())
@@ -101,6 +114,9 @@ class Command(BaseCommand):
             ArticlePage.objects.filter(pk=no_comments_article.pk).update(comments_enabled=False)
             no_comments_article.comments_enabled = False
             no_comments_article.save_revision().publish()
+        ArticlePage.objects.filter(pk=no_comments_article.pk).update(
+            published_date=now - datetime.timedelta(hours=2)
+        )
 
         # About page
         if not AboutPage.objects.child_of(home).filter(slug="about").exists():