#1024 Cancel route kills activity PIDs from the api container's PID namespace

Found while doing #916. api/blueprints/v1/workflows_lifecycle.py cancel_workflow reads process_pid from highway.cancel_workflow_activities() and calls os.killpg(os.getpgid(pid)) / os.kill(pid) inside the api container. The PIDs were recorded by the activity-worker, a different container with its own PID namespace, so the api either hits nothing or signals an unrelated api process that happens to have the same PID (hypercorn workers run as the same user). The kill belongs on the worker that owns the process (e.g. the activity worker polling cancelled activities, or a per-worker task), like the container cleanup moved in #916.