A bunch of small PRs to fix the bugs that surfaced while writing stress tests for the agent-server. OpenHands has already accepted them, but a quick look by a human would also be appreciated. • fix(agent-server): offload upload writes to thread to keep event loop responsive • fix(agent-server): run bash cleanup traps on timeout via process-group SIGTERM • fix(agent-server): bound WebhookSubscriber queue under failed retries • fix(agent-server): propagate subscriber init errors • fix(agent-server): bound initial-state push in subscribe_to_events