Option to pick to redirect back to landing page on bad forward auth request

### Is your feature request related to a problem?

We've had issues where we've had forward authentication setup, and people go to the application, but partway through they bookmark the page, capturing some of the outpost callback details.

ie: They bookmark: https://application.example.com/outpost.goauthentik.io/callback?x-authentik-auth-callback=true&code=XXXXXXXXXXXXXXXXXXXXXXX
Causing them to end up getting this page.

### Describe the solution you'd like

It'd be nice to have a setting to either land the user at that page, or to be able to redirect them back to the default landing page, or to a configurable url.

### Describe alternatives that you've considered

At the moment we're telling users to update their bookmarks to not have any of the callback information.

### Additional context

_No response_

The issue names the /outpost.goauthentik.io/callback path but no files or tests. Start by locating the forward-auth callback handling and its configuration model, then determine how invalid or bookmarked callback requests are handled. Done should provide a selectable default landing page or configurable redirect URL, with behavior covering the existing error page.

Written by the indexing model from the issue text.