- Change test email domain from @example.com to @tests.bookhoard.internal
- This prevents accidental deletion of real user data when self-hosters run tests
- Improve test cleanup: delete ALL users with test domains before each test
- Ensures complete test isolation by cleaning up users from previous tests
- Handles edge cases where tests promote users to admin or modify accounts
The @tests.bookhoard.internal domain is clearly for testing only and
won't conflict with real user emails.