[2.x] add Brevo (Sendinblue) mail driver — core or FoF?

## Brevo (formerly Sendinblue) Mail Driver

Brevo is a popular transactional email provider, particularly common in Europe. Symfony ships a first-party bridge (`symfony/brevo-mailer`) that would make implementation straightforward.

### What would be involved

- New composer dependency: `symfony/brevo-mailer`
- `src/Mail/BrevoDriver.php` implementing `DriverInterface`
- Settings: API key
- `buildTransport()` via `BrevoApiTransport` or `BrevoSmtpTransport`
- Locale keys, unit tests, integration test update

### Question: core or FoF?

Unlike Postmark (where the dep already exists), Brevo would require adding a new composer dependency to core. We're leaning towards a **Friends of Flarum extension** for this one — it keeps core lean and targets a more specific regional audience.

Opinions welcome — particularly from anyone currently using Brevo/Sendinblue with Flarum.