Invitation email templates need invitation context and a configurable subject

## What

invitation-email customization: expose invitation context in the template and allow changing the subject (today hardcoded).

## Want

- Invitation-email customization: expose invitation context in the template
- Allow changing the subject (today hardcoded)

## Ideas this came from

- [#24269 Additional context for invitations](https://github.com/goauthentik/authentik/issues/24269) — Is your feature request related to a problem? The invitation email template only receives url, host, and expires in its template context. This makes it impossib
- [#24964 Invitation email feature should allow changing the subject](https://github.com/goauthentik/authentik/issues/24964) — Is your feature request related to a problem? When sending out invitations via authentik, there is an option to edit the recipients and to create a custom templ

No files or tests are named in the issue. Start by tracing the invitation-email template and the code that sends it; confirm how the current URL, host, and expiry values reach the template. Done means the template receives invitation context and the email subject can be configured instead of remaining hardcoded.

Written by the indexing model from the issue text.