feat(memory): capture user_id at enqueue time for async-safe thread isolation

Add user_id field to ConversationContext and MemoryUpdateQueue.add() so the
user identity is stored explicitly at request time, before threading.Timer
fires on a different thread where ContextVar values do not propagate.
MemoryMiddleware.after_agent() now calls get_effective_user_id() at enqueue
time and passes the value through to updater.update_memory().

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

backend/tests/test_memory_queue_user_isolation.py

@@ -0,0 +1,38 @@
+"""Tests for user_id propagation through memory queue."""
+from unittest.mock import MagicMock, patch
+
+from deerflow.agents.memory.queue import ConversationContext, MemoryUpdateQueue
+
+
+def test_conversation_context_has_user_id():
+    ctx = ConversationContext(thread_id="t1", messages=[], user_id="alice")
+    assert ctx.user_id == "alice"
+
+
+def test_conversation_context_user_id_default_none():
+    ctx = ConversationContext(thread_id="t1", messages=[])
+    assert ctx.user_id is None
+
+
+def test_queue_add_stores_user_id():
+    q = MemoryUpdateQueue()
+    with patch.object(q, "_reset_timer"):
+        q.add(thread_id="t1", messages=["msg"], user_id="alice")
+    assert len(q._queue) == 1
+    assert q._queue[0].user_id == "alice"
+    q.clear()
+
+
+def test_queue_process_passes_user_id_to_updater():
+    q = MemoryUpdateQueue()
+    with patch.object(q, "_reset_timer"):
+        q.add(thread_id="t1", messages=["msg"], user_id="alice")
+
+    mock_updater = MagicMock()
+    mock_updater.update_memory.return_value = True
+    with patch("deerflow.agents.memory.updater.MemoryUpdater", return_value=mock_updater):
+        q._process_queue()
+
+    mock_updater.update_memory.assert_called_once()
+    call_kwargs = mock_updater.update_memory.call_args.kwargs
+    assert call_kwargs["user_id"] == "alice"