Allow setting retentionSecs in backendConfig

Part of #310

src/routes/settings.js

@@ -191,6 +191,7 @@ function setBackupConfig(req, res, next) {
     assert.strictEqual(typeof req.body, 'object');
 
     if (typeof req.body.provider !== 'string') return next(new HttpError(400, 'provider is required'));
+    if (typeof req.body.retentionSecs !== 'number') return next(new HttpError(400, 'retentionSecs is required'));
     if ('key' in req.body && typeof req.body.key !== 'string') return next(new HttpError(400, 'key must be a string'));
 
     settings.setBackupConfig(req.body, function (error) {