chore: review notification settings in the CrowdSec export and check for missing notification files before import #1546

What: the export keeps the notification configuration files in the archive. Excluding them is not an option on its own, because a profile that names a notification with no file in notifications/ stops CrowdSec from loading its configuration (verified with the bundled CrowdSec).
Where: ExportConfig in backend/internal/api/handlers/crowdsec_handler.go; validation in ImportConfig.
Suggested approach: redact the sensitive values inside the notification files on export (keeping the files so profiles stay valid), and add a pre-import check that reports a profile naming a notification with no matching file. Related: #1539 decision to keep these files for now, with the docs asking users to treat an export as private.
Found while planning #1538-#1540. Severity: low.