Introduce artifacts and artifact_versions tables: an append-only,
pass-by-reference store with a stable identity row and immutable
versions that atomically bump current_version. Access is parent-derived
(conversation or workflow run); a CHECK enforces at least one parent and
a nullable team_id is reserved for future team scoping.
Adds the 0025_artifacts migration, ArtifactsRepository with a
parent-scoped safe getter alongside the unscoped one, and repo tests.
Alex
31525621e0