Use HTTP headers to determine OAuth `REDIRECT_URL`

[ace-n]

Right now, we determine it using gcloud run services describe and pass it into the container as an environment variable. This approach may fail for things like custom subdomains.

We should determine the OAuth REDIRECT_URL by examining HTTP headers, since those should account for this.

This is a post-Private Preview ask.

[engelke]

Let's chat for a few minutes when you have time.