Pinging your app every 25 minutes or so is fine, just search for ‘web cron service’ and you’ll find a bunch of free services.
Ah, if you’re automatically uploading/deleting files, then handling them in a local folder in your project might be easier rather than using the assets store. That’s mostly for manual use.
The app sleeps after 5 minutes, and so long as you’ve created an account (logged-in with GH or FB) it’ll stay around.