Send welcome email to new users upon registration

Implement functionality to automatically send a welcome email to users when they register, or use a third-party service to automate this process.

Look for the user registration endpoint in the FastAPI application, likely in a file like `routes/auth.py` or `routes/users.py`. Investigate how emails are currently sent, if at all, and check for any existing email service configuration (e.g., SMTP settings or a third-party API client). 'Done' means a new user receives a welcome email after successful registration, which will require integrating with an email service and adding logic to the registration flow.

Scritto dal modello di indicizzazione a partire dal testo della issue.