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.py

@@ -48,6 +48,7 @@ def test_process_queue_forwards_correction_flag_to_updater() -> None:
         agent_name="lead_agent",
         correction_detected=True,
         reinforcement_detected=False,
+        user_id=None,
     )
 
 
@@ -88,4 +89,5 @@ def test_process_queue_forwards_reinforcement_flag_to_updater() -> None:
         agent_name="lead_agent",
         correction_detected=False,
         reinforcement_detected=True,
+        user_id=None,
     )