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.

The issue points to abandonauth/routers/ui.py line 40 where a relative path is used. Start by examining the Pydantic models and database schema for callback URI validation. Look for existing validation patterns in the codebase. The goal is to enforce a valid URI format and update the UI to use the full URL based on the environment (production vs. dev). Check configuration files for environment-specific settings.

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