v0.4.1

Patch for the statesman conversion (#1): the generator mounts with
changed_at: true, but the 0.4.0 conversion migration never created the
state_changed_at column — the first transition after converting died on
the option's missing-column error.

The conversion now adds the column and backfills it with the honest value
the import already holds: the created_at of the last recorded transition.
Rows that never transitioned stay NULL, exactly like a freshly created
record.

Found by dogfooding the migration path end to end — thanks for reading
this far; the pre-flight checks in the generated migration header still
apply.

Full Changelog: v0.4.0...v0.4.1