PgFlow extensions migration version 4.

Narrows recover_stalled_tasks/1 to tasks pgflow.start_tasks could actually dispatch again: the run must be started and the task's step_states row must be started.

Without the step-state guard, a map step that skips via when_exhausted strands its siblings in started — fail_task archives their messages but never terminalizes the task rows — and the sweep requeued those orphans on an already-completed run. See priv/pgflow_helpers/sql/versions/v04/v04_up.sql for the full rationale.