Fix member unlink: use User update_user action

UnrelateUserWhenArgumentNil used User :update which only accepts :email.
Switch to :update_user with member: nil so manage_relationship clears member_id.

lib/membership/member/changes/unrelate_user_when_argument_nil.ex

@@ -0,0 +1,50 @@
+defmodule Mv.Membership.Member.Changes.UnrelateUserWhenArgumentNil do
+  @moduledoc """
+  When :user argument is present and nil/empty on update_member, unrelate the current user.
+
+  With on_missing: :ignore, manage_relationship does not unrelate when input is nil/[].
+  This change handles explicit unlink (user: nil or user: %{}) by updating the linked
+  User to set member_id = nil. Only runs when the argument key is present (policy
+  ForbidMemberUserLinkUnlessAdmin ensures only admins can pass :user).
+  """
+  use Ash.Resource.Change
+
+  @spec change(Ash.Changeset.t(), keyword(), Ash.Resource.Change.context()) :: Ash.Changeset.t()
+  def change(changeset, _opts, _context) do
+    if unlink_requested?(changeset) do
+      unrelate_current_user(changeset)
+    else
+      changeset
+    end
+  end
+
+  defp unlink_requested?(changeset) do
+    args = changeset.arguments || %{}
+
+    if Map.has_key?(args, :user) or Map.has_key?(args, "user") do
+      user_arg = Ash.Changeset.get_argument(changeset, :user)
+      user_arg == nil or (is_map(user_arg) and map_size(user_arg) == 0)
+    else
+      false
+    end
+  end
+
+  defp unrelate_current_user(changeset) do
+    member = changeset.data
+    actor = Map.get(changeset.context || %{}, :actor)
+
+    case Ash.load(member, :user, domain: Mv.Membership, authorize?: false) do
+      {:ok, %{user: user}} when not is_nil(user) ->
+        # User's :update action only accepts [:email]; use :update_user so
+        # manage_relationship(:member, ..., on_missing: :unrelate) runs and clears member_id.
+        user
+        |> Ash.Changeset.for_update(:update_user, %{member: nil}, domain: Mv.Accounts)
+        |> Ash.update(domain: Mv.Accounts, actor: actor, authorize?: false)
+
+        changeset
+
+      _ ->
+        changeset
+    end
+  end
+end