chore: redact more settings in CrowdSec exports #1566

What: the CrowdSec export now blanks sensitive values in notification files (#1565), but the main config.yaml is still exported as written. Review which of its settings can hold credentials (for example the database section) and blank them the same way, with the import keeping the current values.
Where: backend/internal/crowdsec/export.go, redact.go, redact_merge.go.
Suggested approach: reuse the rewriter with a keep-list for config.yaml, then extend the import merge. Decide how to treat the credentials files that are already preserved by the import.
Severity: low. Found while planning #1551.