Follow Elixir naming conventions by using question mark suffix for boolean
functions. Renamed mail_allowlisted to signup_allowed? and mail_allowlisted_fun
to signup_allowed_fun throughout the codebase.
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
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.
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
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