Fix session cleanup cutoff timezone (#2488)

tests/test_session_manager_cleanup.py

@@ -0,0 +1,34 @@
+from datetime import datetime
+from types import SimpleNamespace
+from unittest.mock import MagicMock
+
+from core.session_manager import SessionManager
+import core.session_manager as SM
+
+
+def _manager_with(sessions=None):
+    manager = SessionManager.__new__(SessionManager)
+    manager.sessions = dict(sessions or {})
+    return manager
+
+
+def test_cleanup_empty_sessions_archives_old_naive_last_accessed(monkeypatch):
+    old_session = SimpleNamespace(
+        id="old-chat",
+        archived=False,
+        last_accessed=datetime(2026, 5, 1, 12, 0, 0),
+        message_count=3,
+        is_important=False,
+    )
+    db = MagicMock()
+    db.query.return_value.all.return_value = [old_session]
+
+    monkeypatch.setattr(SM, "SessionLocal", lambda: db)
+    monkeypatch.setattr(SM, "utcnow_naive", lambda: datetime(2026, 6, 4, 12, 0, 0))
+
+    stats = _manager_with().cleanup_empty_sessions(auto_archive_days=30)
+
+    assert old_session.archived is True
+    assert stats == {"deleted_empty": 0, "archived_old": 1, "total_checked": 1}
+    db.commit.assert_called_once()
+    db.rollback.assert_not_called()