Speed up leaderboards with player-window caches

[colonelpanic8]

Summary

• add disposable player/window leaderboard read models for daily, trailing seven-day, and season scopes
• rebuild cached cohort totals and metric rates transactionally on a configurable recurring schedule
• route standard appearance, event, and stat leaderboards through indexed cache reads with live-query fallback
• simplify the leaderboard UI to total and per-five-minute rankings with window-specific minimum-game defaults
• document throughout that cache rows are rebuildable and never sources of truth

Validation

• PATH=/run/current-system/sw/bin:$PATH just check
• populated temporary-Postgres cache refresh integration test
• 40 leaderboard unit tests