Serve artifact metadata and bytes over HTTP with parent-derived
authorization: conversation-parented artifacts inherit conversation
access (owner, shared_with, or a public share token whose conversation
matches the parent), workflow-run artifacts check run ownership, and
access fails closed when the parent is missing or deleted.
Adds list/get/versions/download/restore routes, an authenticated
storage-agnostic download (sanitized Content-Disposition, 302 to a
short-lived private S3 presigned URL when that strategy is configured),
a generate_presigned_url primitive on the storage base and S3 backend,
and generalizes the tools artifact endpoint for documents/files. Shared
authorization helpers live in a dedicated module used by both surfaces.