feat: prevent join requests with equal mail

lib/membership/join_request/changes/helpers.ex

@@ -16,4 +16,24 @@ defmodule Mv.Membership.JoinRequest.Changes.Helpers do
   end
 
   def actor_id(_), do: nil
+
+  @doc """
+  Extracts the actor's email for display (e.g. reviewed_by_display).
+
+  Supports both atom and string keys for compatibility with different actor representations.
+  """
+  @spec actor_email(term()) :: String.t() | nil
+  def actor_email(nil), do: nil
+
+  def actor_email(actor) when is_map(actor) do
+    raw = Map.get(actor, :email) || Map.get(actor, "email")
+    if is_nil(raw), do: nil, else: actor_email_string(raw)
+  end
+
+  def actor_email(_), do: nil
+
+  defp actor_email_string(raw) do
+    s = raw |> to_string() |> String.trim()
+    if s == "", do: nil, else: s
+  end
 end