Add email service abstraction

Community & Support: [LinkedIn Group](https://www.linkedin.com/groups/14656108/) · [Slack #project-nest](https://owasp.slack.com/archives/project-nest)

Active project leaders: Arkadii Yakovets -- [GitHub](https://github.com/arkid15r/) · [LinkedIn](https://www.linkedin.com/in/arkid15r/) · [Slack](https://owasp.slack.com/team/U060W3NKLTF); Kate Golovanova -- [GitHub](https://github.com/kasya/) · [LinkedIn](https://www.linkedin.com/in/kate-golovanova/) · [Slack](https://owasp.slack.com/team/U07PWB1JZ6Z)

[Contributing](https://github.com/owasp/nest/blob/main/CONTRIBUTING.md) · [Code of Conduct](https://github.com/owasp/nest/blob/main/CODE_OF_CONDUCT.md) · [GSoC Mentors](https://github.com/owasp/nest/blob/main/MENTORS.md)

---

## Description
The backend has no email sending infrastructure. This sub-issue adds an abstract email service layer so the email provider can be swapped easily, along with the default implementation and email settings.
## Changes
- Abstract EmailService base class with send and send_bulk methods
- Default email service implementation using Django's email backend
- Factory function to return the configured email service
- Default sender email setting in base.py
- Unit tests

**Are you going to work on implementing this?**

- [x] Yes
- [ ] No