Hi,
GetCurrentSiteSettings() only works in the context of a web request.
I "think" you might be able to use CacheHelper.GetSiteSettingsFromCache()
I will say that using an HttpModule to do what you are doing is not something I would do and it sounds pretty resource expensive polling the db and creating site user objects from there, it could potentially impact performance or cause weird bugs or side effects. HttpModules are meant to handle events related to http requests not really for running tasks.
Best,
Joe