* add /api/v1/users/{user_id}/stats endpoint
* return 0 stats and add /api/v1/users/{user_id}/stats/{time_frame}
* use utcnow() as modified date for 0 stats
* 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
Andreas Köpf