Restrict User.update_user to admin; allow :update for email only

- Add ActorIsAdmin policy check (admin permission set only)
- User: policy action(:update_user) forbid_unless + authorize_if ActorIsAdmin
- User: primary :update action accept [:email] for non-admin profile edit

lib/accounts/user.ex

@@ -103,6 +103,7 @@ defmodule Mv.Accounts.User do
     #   the specialized :update_user action below.
     update :update do
       primary? true
+      accept [:email]
 
       # Required because custom validation functions (email validation, member relationship validation)
       # cannot be executed atomically. These validations need to query the database and perform
@@ -310,6 +311,14 @@ defmodule Mv.Accounts.User do
       authorize_if expr(id == ^actor(:id))
     end
 
+    # update_user allows :member argument (link/unlink). Only admins may use it to prevent
+    # privilege escalation (own_data could otherwise link to any member and get :linked scope).
+    policy action(:update_user) do
+      description "Only admins can update user with member link/unlink"
+      forbid_unless Mv.Authorization.Checks.ActorIsAdmin
+      authorize_if Mv.Authorization.Checks.ActorIsAdmin
+    end
+
     # UPDATE/DESTROY via HasPermission (evaluates PermissionSets scope)
     policy action_type([:read, :create, :update, :destroy]) do
       description "Check permissions from user's role and permission set"