stop_all() now sweeps /proc for any codai.admin.download_worker processes
and SIGKILLs them after the engines are stopped — including legacy ppid=1
orphans left by an earlier instance that this front never spawned. Orphaned
workers keep holding huggingface_hub's per-blob file lock, which makes the
next re-download deadlock at 0%, so Ctrl-C now guarantees they're cleaned up.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>