Create Backend view for view registrations page

# Story:
AS A Transplant Games registrant,
I WANT to view all my registrations,
SO THAT I can see who I've registered and their status.

## Details:
Create API endpoint to fetch user's registrations with registrant names from joined tables.

**Acceptance Criteria:**
- GET /api/registrations/ endpoint for current user only
- Join Registration with RegistrantDetail models to get names
- Response: registration ID, first_name, last_name, status, created_at
- Handle null/empty names gracefully

**Query approach:**
```python
# Join registrations with registrant_details
Registration.objects.filter(user=request.user).select_related('registrant_details')
# Access via: registration.registrant_details.first_name
```