Maity: Board

hub: attachments core: blob store, migration, upload/get/list/delete, sniffing + image re-encode, quotas and caps, serving headers (§8.7)

tsk-60h8py32k2z4 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).