* add preferred lonely_children extension
* simplify sampling process, lower the probability to 25%
* exclude parents for replies that were recently used
* lonely children := count > 0
* consider only tasks not done for parent exclusion
* increase lonely child sampling probability
* add leaderboard stats, periodic update via fastapi-utils
* count label tasks for assistant and prompter replies
* Daily stats update every 15 mins, simplify leaderboard endpoint
* add indices for some created_date columns
* make user stats update intervals configurable
* make sure intervals are positive