Implement a new `rbox/mail.py` module providing an async email service using `aiosmtplib`, including a background worker queue for non-blocking email dispatch. Integrate the service into the application lifespan (start/stop) and add email sending in three places: welcome email on user registration in `routers/auth.py`, invoice email with line-item details in `billing/invoice_generator.py`, and a test-email admin endpoint in `routers/admin.py`. Add `aiosmtplib==5.0.0` to `requirements.txt`.
