fix: 分组专属倍率查询过滤软删除用户

GetByGroupID 查询 JOIN users 表时未过滤 deleted_at，
导致已删除的用户仍显示在分组专属倍率列表中。

backend/internal/repository/user_group_rate_repo.go

@@ -100,7 +100,7 @@ func (r *userGroupRateRepository) GetByGroupID(ctx context.Context, groupID int6
 	query := `
 		SELECT ugr.user_id, u.email, ugr.rate_multiplier
 		FROM user_group_rate_multipliers ugr
-		JOIN users u ON u.id = ugr.user_id
+		JOIN users u ON u.id = ugr.user_id AND u.deleted_at IS NULL
 		WHERE ugr.group_id = $1
 		ORDER BY ugr.user_id
 	`