feat: email uniqueness constraint between user and member

lib/membership/member.ex

@@ -108,6 +108,10 @@ defmodule Mv.Membership.Member do
     validate present(:last_name)
     validate present(:email)
 
+    # Email uniqueness check for all actions that change the email attribute
+    # Validates that member email is not already used by another (unlinked) user
+    validate Mv.Membership.Member.Validations.EmailNotUsedByOtherUser
+
     # Prevent linking to a user that already has a member
     # This validation prevents "stealing" users from other members by checking
     # if the target user is already linked to a different member