refactor(types): reconcile @specs with their success typings

lib/mv/config.ex

@@ -409,7 +409,7 @@ defmodule Mv.Config do
   @doc """
   Returns the OIDC groups claim name (default "groups"). ENV first, then Settings.
   """
-  @spec oidc_groups_claim() :: String.t() | nil
+  @spec oidc_groups_claim() :: String.t()
   def oidc_groups_claim do
     case env_or_setting("OIDC_GROUPS_CLAIM", :oidc_groups_claim) do
       nil -> "groups"
@@ -492,7 +492,7 @@ defmodule Mv.Config do
   - ENV-only mode (`SMTP_HOST` set): read from ENV `SMTP_PORT`
   - Settings mode: read from Settings only
   """
-  @spec smtp_port() :: non_neg_integer() | nil
+  @spec smtp_port() :: pos_integer() | nil
   def smtp_port do
     if smtp_env_mode?() do
       parse_smtp_port_env(System.get_env("SMTP_PORT"))
@@ -638,9 +638,15 @@ defmodule Mv.Config do
   """
   @spec mail_from_name() :: String.t()
   def mail_from_name do
-    case System.get_env("MAIL_FROM_NAME") do
-      nil -> get_from_settings(:smtp_from_name) || "Mila"
-      value -> trim_nil(value) || "Mila"
+    name =
+      case System.get_env("MAIL_FROM_NAME") do
+        nil -> get_from_settings(:smtp_from_name)
+        value -> trim_nil(value)
+      end
+
+    case name do
+      nil -> "Mila"
+      name -> name
     end
   end