Store and customize email templates, including custom attributes

## What

richer email templating: store/customize templates (DB for k8s) and support custom attributes beyond the stock token-link templates.

## Want

- Richer email templating: store/customize templates (DB for k8s)
- Support custom attributes beyond the stock token-link templates

## Ideas this came from

- [#2505 Add support to store E-Mail templates in database](https://github.com/goauthentik/authentik/issues/2505) — The title says all. Would be nice for k8/helm installs
- [#4311 More generic e-mail templating / custom attributes](https://github.com/goauthentik/authentik/issues/4311) — I want to send information mails to users (especially admins) after password changes, logins, etc. Currently the templating system seems to be focused on valida

Start by reading linked issues #2505 and #4311 to understand the existing requests and unresolved scope. Identify the relevant email-template entry points and storage model in the repository, then define what database-backed templates and custom attributes must support. Done means the scope and acceptance criteria are agreed for both requested capabilities.

Written by the indexing model from the issue text.