Function execute_cron_plugin 

pub async fn execute_cron_plugin(
    plugin: &CrdPlugin,
    crd_kind: &str,
    db_path: Option<&Path>,
) -> Result<()>

Execute a cron plugin (periodic maintenance task).

The plugin receives env: PLUGIN_NAME, PLUGIN_TYPE, CRD_KIND. Returns Ok(()) on success, Err on failure (caller should log, not abort).