Welcome to the community and thanks for your thoughtful questions! 
[1] Yes, Streamlit Community Cloud apps will go to sleep after 12 hours of inactivity (no user traffic), even if the app is public. This means if there are very few transactions, your app may be asleep and unable to receive Stripe webhooks, as incoming webhooks do not count as user traffic and won’t wake the app. This is a limitation of the free Community Cloud service—apps are hibernated to conserve resources and ensure fair usage for all users. See the official documentation and forum discussions for details: Streamlit Docs, Forum 1, Forum 2.
[2] Currently, there is no paid tier or subscription on Streamlit Community Cloud that allows you to keep your app awake 24/7. If you need your app to always be online to receive webhooks reliably, you should consider deploying to a commercial cloud provider like AWS, Google Cloud, or Heroku, where you can control uptime and background process handling. See Forum 1 and Forum 2 for more on this.
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