ruff: enable DTZ (flake8-datetimez)

Fixes 44 violations — naive datetime usage replaced with tz-aware
equivalents throughout production and test code:
- datetime.now() → timezone.now() (Django) or datetime.now(tz=UTC)
- datetime.fromtimestamp() → datetime.fromtimestamp(ts, tz=UTC)
- datetime.date.today() → timezone.now().date()
- datetime.datetime(...) constructors → tzinfo=UTC in tests
- UP017 auto-converted datetime.timezone.utc → datetime.UTC (py3.11+)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/documents/tests/test_api_documents.py

@@ -265,7 +265,7 @@ class TestDocumentApi(DirectoriesMixin, ConsumeTaskMixin, APITestCase):
             created=date(2023, 1, 1),
         )
 
-        created_datetime = datetime.datetime(2023, 2, 1, 12, 0, 0)
+        created_datetime = datetime.datetime(2023, 2, 1, 12, 0, 0, tzinfo=datetime.UTC)
         response = self.client.patch(
             f"/api/documents/{doc.pk}/",
             {"created": created_datetime},