allow explicitly providing redirect_uri for github sign in

when trying to deploy keystatic on our infrastructure, i am running into an issue with the redirect_uri setting for github sign in.

the callback uri in the github app is configured to use the /api/keystatic/github/oauth/callback route on the domain the app is deployed to, however the redirect_uri actually used by keystatic is using a hostname internal to our cluster infra. (in our case, the correct hostname would be only on x-forwarded-host header)

which means i cannot sign in, but instead see this error:

An error occurred when trying to authenticate with GitHub:
The redirect_uri MUST match the registered callback URL for this application.

the redirect_uri query param is constructed here by keystatic:

https://github.com/Thinkmill/keystatic/blob/bcc3c69638c3affd758215289f572ac22afadfa5/packages/keystatic/src/api/generic.ts#L375-L378

allowing to set the redirect url explicitly via environment variable would work around this issue i think.

Start in packages/keystatic/src/api/generic.ts at the redirect_uri construction around lines 375-378, and trace how the GitHub OAuth configuration is read. Add an explicit configuration path for the deployment-provided callback URL, then verify that sign-in sends the configured public URL instead of the internal hostname.