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>