Make a staging server

[kcarnold]

staging.thoughtful-ai.com has been set up to forward to port 19573.

We need to configure Jenkins to:

• only deploy prod when we tag a release
• use 19573 for a staging server when we get a push to main

Posthog should probably know whether it's coming from staging or prod. We may need to inject a variable into build of whether we're building staging or prod—or just look at the URL / port.