refactor: centralize SIGNUP_ALLOWLIST_EMAILS constant definition

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>

lib/basic_signup_allowlist.ex

@@ -1,10 +1,17 @@
 defmodule BasicSignupAllowlist do
   import FunCore.BasicSignupAllowlist
 
+  @signup_allowlist_emails "SIGNUP_ALLOWLIST_EMAILS"
+
   @moduledoc """
   Checks if an email address is allowed based on the SIGNUP_ALLOWLIST_EMAILS env variable.
   """
 
+  @doc """
+  Returns the name of the environment variable used for the signup allowlist.
+  """
+  def signup_allowlist_emails, do: @signup_allowlist_emails
+
   @doc """
   Checks if an email address is allowed based on the SIGNUP_ALLOWLIST_EMAILS env variable.
 
@@ -27,7 +34,7 @@ defmodule BasicSignupAllowlist do
 
   """
   def mail_allowlisted(email) do
-    env_value = System.get_env("SIGNUP_ALLOWLIST_EMAILS")
+    env_value = System.get_env(@signup_allowlist_emails)
     mail_allowlisted_fun(env_value, email)
   end
 end

test/basic_signup_allowlist_test.exs

@@ -3,21 +3,21 @@ defmodule BasicSignupAllowlistTest do
   doctest BasicSignupAllowlist
 
   defp allow_signups_for(allowlist) do
-    System.put_env("SIGNUP_ALLOWLIST_EMAILS", allowlist)
+    System.put_env(BasicSignupAllowlist.signup_allowlist_emails(), allowlist)
   end
 
   describe "setup in describe block" do
     setup do
       on_exit(fn ->
         # resetting values did not work, so 'just' set it to the effect without this module configured.
-        System.put_env("SIGNUP_ALLOWLIST_EMAILS", "*")
+        System.put_env(BasicSignupAllowlist.signup_allowlist_emails(), "*")
       end)
 
       :ok
     end
 
     test "When not set, not allowlisted" do
-      System.delete_env("SIGNUP_ALLOWLIST_EMAILS")
+      System.delete_env(BasicSignupAllowlist.signup_allowlist_emails())
       refute BasicSignupAllowlist.mail_allowlisted("joe@example.com")
     end