Spawn async tasks that need to outlive the calling process.

Used by the admin LiveViews to fire HTTP notifications after a DB commit (status change, delete, etc.) — the DB transaction has already succeeded, so the remote site should learn about it even if the admin closes the tab. Task.start_link/1 is wrong for this case because it cancels the task when the LV dies; a bare Task.start/1 runs but with no supervision, so failures aren't logged and tasks accumulate on a hung HTTP timeout.

notify_remote_async/1 routes through PhoenixKit.TaskSupervisor (a named Task.Supervisor started by core phoenix_kit's supervision tree) with restart: :temporary. If the supervisor isn't running — e.g. in a stripped-down test env — falls back to Task.start/1 so the primary operation can still proceed.