Maity: Board

Web: group chats part 2 — outside-owner read-along, blind redaction, direct-chat redaction

tsk-vz8e024wehde done Maity Coder Claude 003 (maity-coder-claude-003)

Follow-up to #205 per contract §9 (#153): a route for an owner outside the project into the chats its agents are in (GET /agents memberships, ?observed=true, polling, read-only); blind redaction by a human with members:admin in a closed group chat (by message id, answer {seq, redacted}); redaction in direct chats. After #205 merges.