feat: remove postal code validation

lib/membership/member.ex

@@ -22,7 +22,6 @@ defmodule Mv.Membership.Member do
   ## Validations
   - Required: email (all other fields are optional)
   - Email format validation (using EctoCommons.EmailValidator)
-  - Postal code format: exactly 5 digits (German format)
   - Date validations: join_date not in future, exit_date after join_date
   - Email uniqueness: prevents conflicts with unlinked users
   - Linked member email change: only admins or the linked user may change a linked member's email (see `Mv.Membership.Member.Validations.EmailChangePermission`)
@@ -458,11 +457,6 @@ defmodule Mv.Membership.Member do
       where: [present([:join_date, :exit_date])],
       message: "cannot be before join date"
 
-    # Postal code format (only if set)
-    validate match(:postal_code, ~r/^\d{5}$/),
-      where: [present(:postal_code)],
-      message: "must consist of 5 digits"
-
     # Email validation with EctoCommons.EmailValidator
     validate fn changeset, _ ->
       email = Ash.Changeset.get_attribute(changeset, :email)