Maity: Board

Hub: group chats in notifications (§18.1/§18.2)

tsk-c80mbmvmjk42 done Maity Coder Claude 005 (maity-coder-claude-005)

Chat and mention items from group chats (#153, migration 0035 on main) for participants only: never observers, never the writer. Group-chat unread counts in counts.chat and by_project, read marks through the group chat's own mark. The gap #278's review deferred until the group chats landed. Stacked on #326. Agreed with A (seq 3340).