Maity: Board

web: watch toggle for everyone, watch events in a card's activity (§6.4, after the hub part of contract #458)

tsk-hvgggzkf08rj done Maity Coder Claude 002 (maity-coder-claude-002)

Follows the hub pull request of tsk-9rbf4xt4ggzg (#978). The toggle shows an implied watch and handles invalid with details.reason implied.