refactor: rename mail_allowlisted to signup_allowed?

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>

lib/basic_signup_allowlist.ex

@@ -26,15 +26,15 @@ defmodule BasicSignupAllowlist do
   ## Examples
 
       iex> System.delete_env("SIGNUP_ALLOWLIST_EMAILS")
-      iex> BasicSignupAllowlist.mail_allowlisted("joe@example.com")
+      iex> BasicSignupAllowlist.signup_allowed?("joe@example.com")
       false
       iex> System.put_env("SIGNUP_ALLOWLIST_EMAILS","*")
-      iex> BasicSignupAllowlist.mail_allowlisted("joe@example.com")
+      iex> BasicSignupAllowlist.signup_allowed?("joe@example.com")
       true
 
   """
-  def mail_allowlisted(email) do
+  def signup_allowed?(email) do
     env_value = System.get_env(@signup_allowlist_emails)
-    mail_allowlisted_fun(env_value, email)
+    signup_allowed_fun(env_value, email)
   end
 end

lib/fun_core/basic_signup_allowlist.ex

@@ -9,7 +9,7 @@ defmodule FunCore.BasicSignupAllowlist do
     |> Enum.map(&normalize/1)
   end
 
-  def mail_allowlisted_fun(signups_allowed, email_received) do
+  def signup_allowed_fun(signups_allowed, email_received) do
     case signups_allowed do
       nil -> false
       "*" -> true