Implement Signal Propagation Configuration Option

**Overview**
Add a configuration option `DAGU_SIGNAL_PROPAGATION` to enable signal propagation from the Dagu server/scheduler to running DAG processes.

**Configuration Syntax in `admin.yaml`**

```yaml
signalHandling:
enablePropagation: true
```

**environment variable**
```bash
DAGU_SIGNAL_PROPAGATION=1
```

**Additional Context**
https://discord.com/channels/1095289480774172772/1095291629344149544/1241031496698040392

Start with the admin.yaml signalHandling configuration and trace how configuration and environment variables are parsed by the server or scheduler. Then follow signal handling for running DAG processes; the work is done when enablePropagation and DAGU_SIGNAL_PROPAGATION=1 consistently enable propagation from the server or scheduler.

Written by the indexing model from the issue text.