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