feat: make member emails unique

lib/membership/member.ex

@@ -247,4 +247,9 @@ defmodule Mv.Membership.Member do
     # The relationship is optional (allow_nil? true by default)
     has_one :user, Mv.Accounts.User
   end
+
+  # Define identities for upsert operations
+  identities do
+    identity :unique_email, [:email]
+  end
 end