apply backup retention policy

part of #441

src/routes/settings.js

@@ -107,7 +107,7 @@ function setBackupConfig(req, res, next) {
     if ('acceptSelfSignedCerts' in req.body && typeof req.body.acceptSelfSignedCerts !== 'boolean') return next(new HttpError(400, 'format must be a boolean'));
 
     if (!req.body.retentionPolicy || typeof req.body.retentionPolicy !== 'object') return next(new HttpError(400, 'retentionPolicy is required'));
-    if (typeof req.body.retentionPolicy.keepWithin !== 'number') return next(400, 'keepWithin is required');
+    if (typeof req.body.retentionPolicy.keepWithinSecs !== 'number') return next(400, 'keepWithinSecs is required');
 
     // testing the backup using put/del takes a bit of time at times
     req.clearTimeout();