Study stats services

Folder: services/user-study-stats/

File	Role
`study-stats.service.ts`	Dashboard queries (rollups, streaks)
`study-stats-window.ts`	Compute date ranges (today, week, …)
`record-practice-attempt-analytics.ts`	Write attempt event + update rollups
`record-recent-practice-subtopic.ts`	Update “recent” pointer after practice

Called from user routes and practice routes after submissions.