Member: validate configurable and Vereinfacht-required fields

Add validation for required member fields from settings and for
Vereinfacht-required fields when integration is configured.

lib/membership/member.ex

@@ -544,6 +544,43 @@ defmodule Mv.Membership.Member do
              "Unable to validate required custom fields. Please try again or contact support."}
       end
     end
+
+    # Validate member fields that are marked as required in settings or by Vereinfacht
+    validate fn changeset, _context ->
+      case Mv.Membership.get_settings() do
+        {:ok, settings} ->
+          required_config = settings.member_field_required || %{}
+          normalized = VisibilityConfig.normalize(required_config)
+          vereinfacht_required? = Mv.Config.vereinfacht_configured?()
+
+          required_fields =
+            Enum.filter(Mv.Constants.member_fields(), fn field ->
+              field == :email ||
+                (vereinfacht_required? && Mv.Constants.vereinfacht_required_field?(field)) ||
+                Map.get(normalized, field, false)
+            end)
+
+          missing =
+            Enum.filter(required_fields, fn field ->
+              value = Ash.Changeset.get_attribute(changeset, field)
+              not member_field_value_present?(field, value)
+            end)
+
+          if Enum.empty?(missing) do
+            :ok
+          else
+            # Return first missing field error (Ash shows one at a time per field)
+            field = hd(missing)
+
+            {:error,
+             field: field, message: Gettext.dgettext(MvWeb.Gettext, "default", "can't be blank")}
+          end
+
+        {:error, _} ->
+          # If settings cannot be loaded, skip this validation (e.g. bootstrap)
+          :ok
+      end
+    end
   end
 
   attributes do
@@ -1420,4 +1457,14 @@ defmodule Mv.Membership.Member do
   defp value_present?(_value, :email), do: false
 
   defp value_present?(_value, _type), do: false
+
+  # Used by member-field-required validation (settings-driven required fields)
+  defp member_field_value_present?(_field, nil), do: false
+
+  defp member_field_value_present?(_, value) when is_binary(value),
+    do: String.trim(value) != ""
+
+  defp member_field_value_present?(_, %Date{}), do: true
+  defp member_field_value_present?(_, value) when is_struct(value, Date), do: true
+  defp member_field_value_present?(_, _), do: false
 end