Enforce schema in Callback URIs and use full URI in abandon auth UI

## Summary

Callback URIs can be set to any arbitrary value, including relative paths or just nonsense. Currently the AbandonAuth UI for developer dashboards uses a [relative path](https://github.com/AbandonTech/abandonauth/blob/main/abandonauth/routers/ui.py#L40) for the callback URI. This should be changed to use the full URL in the specific environment. I.e in production we should be using `auth.abandontech.cloud/ui`, in dev it should use `localhost:{port}/ui` where port is the actual port the application is being locally deployed with.

## Acceptance Criteria

Callback URIs have validation either in the database, Pydantic models, or both that forces them to be in the form of a valid URI, therefore disallowing relative paths or random strings.
The AbandonAuth UI must also use the new, correct format for its own redirect to the AbandonAuth login page.