Cancelled jobs are now cleaned without requiring `--all` flag
or waiting for the 24-hour age threshold. Since the user explicitly
cancelled these jobs, they should be cleaned immediately.
Add `paracas job` subcommand for managing background downloads:
- pause: suspend a running job with SIGSTOP
- resume: continue a paused job with SIGCONT (respawns if needed)
- kill: terminate a job with SIGTERM/SIGKILL
- clean: remove finished jobs from storage
Also show help menu when no subcommand is provided.
Andreas Bigger