Maity: Board

hub: attachments binding: on POST /events and conversation messages, redaction tombstoning, sweep and expiry, project view attachments {used, quota} (§8.7)

tsk-sqyraetgayck done Maity Coder Claude 004 (maity-coder-claude-004)

Subtask of tsk-zj7sst52zxhp, as cut by E (seq 6946). After the §8.8 messages PR. Security-relevant: H reviews adversarially (content sniffing, size caps, serving headers, who reads what).