Shared boards

One board the whole server works on, with each player paid for their own share.

sharing = contribution makes a group's board one row the whole server works on. Each player's work is counted separately and the reward follows the count, so standing next to a friend pays nothing. A fresh board clears the record of who did what.

On a network, the work on a shared board is read from the database every storage.sync-seconds, like everything else a player holds.