add typesecs and docstrings

lib/basic_signup_allowlist.ex

@@ -10,6 +10,7 @@ defmodule BasicSignupAllowlist do
   @doc """
   Returns the name of the environment variable used for the signup allowlist.
   """
+  @spec signup_allowlist_emails() :: String.t()
   def signup_allowlist_emails, do: @signup_allowlist_emails
 
   @doc """
@@ -33,6 +34,7 @@ defmodule BasicSignupAllowlist do
       true
 
   """
+  @spec signup_allowed?(String.t()) :: boolean()
   def signup_allowed?(email) do
     env_value = System.get_env(@signup_allowlist_emails)
     signup_allowed_fun(env_value, email)

lib/fun_core/basic_signup_allowlist.ex

@@ -1,14 +1,30 @@
 defmodule FunCore.BasicSignupAllowlist do
+  @moduledoc """
+  Functional core for email allowlist checking logic.
+  """
+
+  @doc """
+  Normalizes an email address by trimming whitespace and converting to lowercase.
+  """
+  @spec normalize(String.t()) :: String.t()
   def normalize(email) do
     email |> String.trim() |> String.downcase()
   end
 
+  @doc """
+  Converts a comma-separated string of email addresses into a normalized list.
+  """
+  @spec addresses_as_list(String.t()) :: [String.t()]
   def addresses_as_list(addresses_str) do
     addresses_str
     |> String.split(",")
     |> Enum.map(&normalize/1)
   end
 
+  @doc """
+  Checks if an email is allowed based on the allowlist configuration.
+  """
+  @spec signup_allowed_fun(String.t() | nil, String.t()) :: boolean()
   def signup_allowed_fun(signups_allowed, email_received) do
     case signups_allowed do
       nil -> false