Fix databi backfill active day formatting

services/statistics-service/cmd/backfill-last7/main.go

@@ -356,7 +356,7 @@ func rebuildRegistrations(ctx context.Context, db *sql.DB, state *rebuildState,
 func rebuildActive(ctx context.Context, db *sql.DB, state *rebuildState, app, startDay, endDay string) error {
 	// 活跃的原始房间事件已经被实时消费进 stat_user_day_activity；离线回填只重刷该表的国家/区域快照，避免在大窗口内扫描房间流水。
 	rows, err := db.QueryContext(ctx, `
-		SELECT stat_day, user_id, MIN(first_active_at_ms)
+		SELECT DATE_FORMAT(stat_day, '%Y-%m-%d'), user_id, MIN(first_active_at_ms)
 		FROM stat_user_day_activity
 		WHERE app_code = ? AND stat_day BETWEEN ? AND ?
 		GROUP BY stat_day, user_id`,