Refactor sponsorship email to send asynchronously via Celery

[d-mahi14]

Fix #242
@Mariatta

This PR refactors the sponsorship email system so that emails are sent asynchronously using Celery instead of directly in the signals.

Changes included:

1. Setup Celery task infrastructure in tasks.py for sponsorship emails.
2. Moved sponsorship onboarding and progress update emails to Celery tasks:
   send_internal_sponsor_onboarding_email_task
   send_internal_sponsor_progress_update_email_task
3. Updated settings to configure Django email backend for local testing (console backend).
4. Added retry logic in tasks to handle temporary failures.
5. Made ALLOWED_HOSTS environment variable safe for Docker and local development.

How to test

1. Start docker: docker compose up --build
2. Run a task manually: python manage.py shell
   from sponsorship.tasks import send_internal_sponsor_onboarding_email_task
   send_internal_sponsor_onboarding_email_task.delay(profile_id=1)
3. Verify email output in MailDev console at http://localhost:1080

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[d-mahi14]

@Mariatta can you please check this pr?