chore: CrowdSec config import does not restore installed hub item links #1538

What: hub items installed through cscli are symlinks in the CrowdSec config tree. The config export skips symlinks, and a config import clears the live tree, so installed hub item links are not restored by an import (a live install had 66 of them). CrowdSec can reinstall them, but until it does the restored configuration is missing items.
Where: ExportConfig and ImportConfig in backend/internal/api/handlers/crowdsec_handler.go; ClearConfig / copyTree in backend/internal/crowdsec/backup.go.
Suggested approach: keep live links across an import (like other server-local state), or record installed items in the export and re-install them after import; decide whether config/hub/ should stay in the export.
Found while planning #1532-#1534. Severity: medium.