Add email-change permission validation for linked members

Only admins or the linked user may change a linked member's email.
- New validation EmailChangePermission (uses Actor.admin?, Loader.get_linked_user).
- Register on Member update_member; docs and gettext.

lib/mv/membership/member/validations/email_change_permission.ex

@@ -0,0 +1,69 @@
+defmodule Mv.Membership.Member.Validations.EmailChangePermission do
+  @moduledoc """
+  Validates that only admins or the linked user may change a linked member's email.
+
+  This validation runs on member update when the email attribute is changing.
+  It allows the change only if:
+  - The member is not linked to a user, or
+  - The actor has the admin permission set (via `Mv.Authorization.Actor.admin?/1`), or
+  - The actor is the user linked to this member (actor.member_id == member.id).
+
+  This prevents non-admins from changing another user's linked member email,
+  which would sync to that user's account and break email synchronization.
+
+  No system-actor fallback: missing actor is treated as not allowed.
+  """
+  use Ash.Resource.Validation
+  use Gettext, backend: MvWeb.Gettext, otp_app: :mv
+
+  alias Mv.Authorization.Actor
+  alias Mv.EmailSync.Loader
+
+  @doc """
+  Validates that the actor may change the member's email when the member is linked.
+
+  Only runs when the email attribute is changing (checked inside). Skips when
+  member is not linked. Allows when actor is admin or owns the linked member.
+  """
+  @impl true
+  def validate(changeset, _opts, context) do
+    if Ash.Changeset.changing_attribute?(changeset, :email) do
+      validate_linked_member_email_change(changeset, context)
+    else
+      :ok
+    end
+  end
+
+  defp validate_linked_member_email_change(changeset, context) do
+    linked_user = Loader.get_linked_user(changeset.data)
+
+    if is_nil(linked_user) do
+      :ok
+    else
+      actor = resolve_actor(changeset, context)
+      member_id = changeset.data.id
+
+      if Actor.admin?(actor) or actor_owns_member?(actor, member_id) do
+        :ok
+      else
+        msg =
+          dgettext("default", "Only administrators can change email for members linked to users")
+
+        {:error, field: :email, message: msg}
+      end
+    end
+  end
+
+  # Ash stores actor in changeset.context.private.actor; validation context also has .actor
+  defp resolve_actor(changeset, context) do
+    get_in(changeset.context || %{}, [:private, :actor]) ||
+      (context && Map.get(context, :actor))
+  end
+
+  defp actor_owns_member?(nil, _member_id), do: false
+
+  defp actor_owns_member?(actor, member_id) do
+    actor_member_id = Map.get(actor, :member_id) || Map.get(actor, "member_id")
+    actor_member_id == member_id
+  end
+end