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/tests/test_nightly_e2e_playwright.py

@@ -32,7 +32,7 @@ def test_nightly_playwright_journey() -> None:
 
         article_url = article_href if article_href.startswith("http") else f"{base_url}{article_href}"
         page.goto(article_url, wait_until="networkidle")
-        expect(page.get_by_role("heading", name="Comments")).to_be_visible()
+        expect(page.get_by_role("heading", name="Comments", exact=True)).to_be_visible()
         expect(page.get_by_role("button", name="Post comment")).to_be_visible()
 
         page.goto(f"{base_url}/feed/", wait_until="networkidle")