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_custom_fields.py

@@ -1,5 +1,5 @@
+import datetime
 import json
-from datetime import date
 from unittest import mock
 from unittest.mock import ANY
 
@@ -456,7 +456,7 @@ class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):
             },
         )
 
-        date_value = date.today()
+        date_value = datetime.datetime.now(tz=datetime.UTC).date()
 
         resp = self.client.patch(
             f"/api/documents/{doc.id}/",
@@ -618,7 +618,7 @@ class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):
             data_type=CustomField.FieldDataType.DATE,
         )
 
-        date_value = date.today()
+        date_value = datetime.datetime.now(tz=datetime.UTC).date()
 
         resp = self.client.patch(
             f"/api/documents/{doc.id}/",