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>

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