Replace GROUP BY user_id,username with GROUP BY user_id and a
correlated subquery that picks the latest username per user.
This fixes the case where a user changes their Discord display
name and appears as two separate leaderboard entries.
Affected queries: GetLeaderboard, GetUserStats, GetStatsInRange,
GetYearlyLeaderboard, GetUserYearlyStats.
Also keeps the write-time username sync in AddLog for long-term
data cleanup.
Signed-off-by: Blake Ridgway <blake@blakeridgway.com>
d858d04e84