Forward-auth single application across multiple hosts

## What

forward-auth (single application) across hosts: Caddy “not found” and multiple domains on one single-app provider.

## Want

- Forward-auth (single application) across hosts: Caddy “not found”
- Multiple domains on one single-app provider

## Ideas this came from

- [#12930 Error not found when I set up forward_auth (single application) with Caddy](https://github.com/goauthentik/authentik/issues/12930) — Describe your question I created a provider using the wizard. - Under Application Details: Name: Mailu Slug: mailu - Under Provider Type, I chose Forward Auth (
- [#16399 Forward Auth (single application) Multiple domain](https://github.com/goauthentik/authentik/issues/16399) — When setting up Forward Auth for a single application you need to create a new provider per separate domain. Which in general is good practice. However when dea

Start by reading the linked issues #12930 and #16399, then inspect the current forward-auth single-application behavior. Define how Caddy's “not found” case and multiple domains should work before identifying the relevant implementation and tests; done means both requested scenarios are supported.

Written by the indexing model from the issue text.