v2.0.0

Sources can now drift out from under a published set and the service notices. Re-hash a source on demand (or re-register it with new content, or let the scheduler do it on an interval) and every step that cites it is flagged stale; GET /instruction-sets/{id}/drift shows the open and resolved flags, and publish is refused until an expert re-verifies the steps or edits them.

An edit only refreshes source hashes on the steps it actually changed, so fixing an unrelated step does not quietly clear a flag. Migration 0002 adds drift_flags and sources.last_checked_at; the suite now also round-trips every Alembic revision. 38 tests.