Renew the Datum log token before it expires

The Datum bearer token is fetched once during `pilot setup logs` and
written to system/fluent-bit/token.env. Nothing refreshes it; TTL is 1
year, so shipping silently stops after a year.

Fix: on `setup logs`, install a weekly cron (reuse CronManager +
cron_module_command like registry_cache) that re-fetches via
CentralClient.log_token() and, if it changed, rewrites token.env and
restarts the service. Skip when --token was passed or Central is not
configured. --remove drops the cron.

Files: pilot/managers/fluentbit/__init__.py, pilot/commands/setup/logs.py,
tests/pilot/managers/test_fluentbit.py
Parent: #438