Fetch an attachment retained inline with a document

Serve the bytes of an attachment retained as inline content. The id is the one inside a placeholder token, and is returned on attachments[].url by recall and by the document/chunk/memory reads — so an agent can show or reason over the original behind an attachment-derived fact.

Bytes are served with the Content-Type the caller declared at retain. Access is authorized against the bank; a missing attachment and an invisible bank both return 404, so the endpoint cannot be used to probe what a bank holds.