[OAuth] Celery Task for Sending Welcome Emails

- **Task:** Create a **Celery task** that sends a welcome email on successful registration or login.
- **Steps:**
  1. Add Celery task logic to **send an email** to the user.
  2. Configure the **email template** with the user's details.
- **Deliverable:** Email notifications sent via Celery.

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US #93 