Define environment variable name as a module attribute in BasicSignupAllowlist
and expose it via signup_allowlist_emails/0 function to eliminate duplication
across lib and test files.
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Co-Authored-By: Claude <noreply@anthropic.com>
Prefactoring, from TDDAYMI to imperative shell. In the next step we
need the implementation from the functional core in the imperative
shell, to pass in the environment variable.