Adds received_at, in_progress_started_at, qc_started_at,
ready_to_ship_at, shipped_at - snapshotted on state change via
write() override using super().write() to avoid recursion. Required
for the vertical-timeline rendering on the job detail page (Phase 3).
Idempotent: re-transitioning to a state already-stamped does not
overwrite the original timestamp.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
9d58f5f61e