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/blog/tests/test_admin_experience.py

@@ -1,3 +1,5 @@
+from datetime import timedelta
+
 import pytest
 from django.test import override_settings
 from django.utils import timezone
@@ -32,7 +34,7 @@ def test_published_date_preserved_when_explicitly_set(home_page):
     index = ArticleIndexPage(title="Articles", slug="articles")
     home_page.add_child(instance=index)
     author = AuthorFactory()
-    custom_date = timezone.now() - timezone.timedelta(days=30)
+    custom_date = timezone.now() - timedelta(days=30)
     article = ArticlePage(
         title="Custom Date",
         slug="custom-date",
@@ -60,7 +62,7 @@ def test_homepage_orders_articles_by_published_date(home_page):
         author=author,
         summary="s",
         body=[("rich_text", "<p>body</p>")],
-        published_date=timezone.now() - timezone.timedelta(days=10),
+        published_date=timezone.now() - timedelta(days=10),
     )
     index.add_child(instance=older)
     older.save_revision().publish()
@@ -94,7 +96,7 @@ def test_article_index_orders_by_published_date(home_page, rf):
         author=author,
         summary="s",
         body=[("rich_text", "<p>b</p>")],
-        published_date=timezone.now() - timezone.timedelta(days=5),
+        published_date=timezone.now() - timedelta(days=5),
     )
     index.add_child(instance=old)
     old.save_revision().publish()