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>

test/basic_signup_allowlist_test.exs

@@ -18,12 +18,12 @@ defmodule BasicSignupAllowlistTest do
 
     test "When not set, not allowlisted" do
       System.delete_env(BasicSignupAllowlist.signup_allowlist_emails())
-      refute BasicSignupAllowlist.mail_allowlisted("joe@example.com")
+      refute BasicSignupAllowlist.signup_allowed?("joe@example.com")
     end
 
     test "When set to star, allowlisted" do
       allow_signups_for("*")
-      assert BasicSignupAllowlist.mail_allowlisted("joe@example.com")
+      assert BasicSignupAllowlist.signup_allowed?("joe@example.com")
     end
   end
 end

test/fun_core/basic_signup_allowlist_test.exs

@@ -16,25 +16,25 @@ defmodule FunCore.BasicSignupAllowlistTest do
 
   describe "Not allowlisted when allowed list is" do
     test "not set" do
-      refute(mail_allowlisted_fun(nil, "joe@example.com"))
+      refute(signup_allowed_fun(nil, "joe@example.com"))
     end
 
     test "empty" do
-      refute(mail_allowlisted_fun("j", "joe@example.com"))
+      refute(signup_allowed_fun("j", "joe@example.com"))
     end
   end
 
   describe "Allowlisted when" do
     test "*" do
-      assert(mail_allowlisted_fun("*", "jane@example.com"))
+      assert(signup_allowed_fun("*", "jane@example.com"))
     end
 
     test "Multiple set and one match" do
-      assert(mail_allowlisted_fun("joe@example.com, jane@example.com", "jane@example.com"))
+      assert(signup_allowed_fun("joe@example.com, jane@example.com", "jane@example.com"))
     end
 
     test "Matches with different casings" do
-      assert(mail_allowlisted_fun("joe@Example.com, jane@example.com", "Joe@example.com"))
+      assert(signup_allowed_fun("joe@Example.com, jane@example.com", "Joe@example.com"))
     end
   end
 end