Add scheduled/daemon mode for continuous cleanup

## Context
Users currently run `zclean` manually. A daemon mode that periodically scans and cleans would be more convenient.

## What to do
Add `zclean --daemon` or `zclean --interval 60` that runs the scan every N seconds in the background.

## Considerations
- Should log to a file
- Graceful shutdown on SIGTERM/SIGINT
- Option to set interval

## Difficulty: Medium