feat(memory): thread user_id through memory updater layer

Add `user_id` keyword-only parameter to all public updater functions
(_save_memory_to_file, get_memory_data, reload_memory_data, import_memory_data,
clear_memory_data, create/delete/update_memory_fact) and regular keyword param
to MemoryUpdater.update_memory + update_memory_from_conversation, propagating
it to every storage load/save/reload call.

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

backend/tests/test_memory_updater_user_isolation.py

@@ -0,0 +1,29 @@
+"""Tests for user_id propagation in memory updater."""
+from unittest.mock import MagicMock, patch
+
+from deerflow.agents.memory.updater import get_memory_data, clear_memory_data, _save_memory_to_file
+
+
+def test_get_memory_data_passes_user_id():
+    mock_storage = MagicMock()
+    mock_storage.load.return_value = {"version": "1.0"}
+    with patch("deerflow.agents.memory.updater.get_memory_storage", return_value=mock_storage):
+        get_memory_data(user_id="alice")
+        mock_storage.load.assert_called_once_with(None, user_id="alice")
+
+
+def test_save_memory_passes_user_id():
+    mock_storage = MagicMock()
+    mock_storage.save.return_value = True
+    with patch("deerflow.agents.memory.updater.get_memory_storage", return_value=mock_storage):
+        _save_memory_to_file({"version": "1.0"}, user_id="bob")
+        mock_storage.save.assert_called_once_with({"version": "1.0"}, None, user_id="bob")
+
+
+def test_clear_memory_data_passes_user_id():
+    mock_storage = MagicMock()
+    mock_storage.save.return_value = True
+    with patch("deerflow.agents.memory.updater.get_memory_storage", return_value=mock_storage):
+        clear_memory_data(user_id="charlie")
+        # Verify save was called with user_id
+        assert mock_storage.save.call_args.kwargs["user_id"] == "charlie"