Add on-demand remote refresh command

[icey-max]

Summary

• add ./icloudctl refresh to request an immediate remote metadata crawl from the running service
• wake the background refresh loop with SIGUSR1 instead of waiting for the scheduled interval
• document the command and add unit coverage for the refresh trigger
• quote generated systemd service paths so checkouts with spaces work correctly

Tests

• .venv/bin/python -m unittest -v
• bash -n icloudctl