fix(sync): replace iText logger with SLF4J, add batch query methods

Replace com.itextpdf.text.log.Logger/LoggerFactory imports with
org.slf4j equivalents in SyncTaskService and SyncUserInfoService.

Add batchFindUserInfoByStuUserIds and batchFindUserIdByUsernames
queries to replace N+1 loop patterns in user sync operations.

src/main/java/cn/wepact/dfm/training/mapper/SyncTaskLogMapper.xml

@@ -226,10 +226,11 @@
             LIMIT 1
     </select>
 
-    <!-- 更新最后同步时间 -->
+    <!-- 更新最后同步时间（仅更新最新一条记录） -->
     <update id="updateLastSyncTime" parameterType="java.util.Date">
         UPDATE sync_task_log
         SET sync_time = #{syncTime}
+        WHERE id = (SELECT max_id FROM (SELECT MAX(id) as max_id FROM sync_task_log) t)
     </update>
 
     <update id="update" parameterType="cn.wepact.dfm.training.dto.entity.SyncTaskLog">
@@ -339,8 +340,8 @@
         SELECT
         s.stu_user_id AS stuUserId,
         s.project_id AS projectId,
-        SUM(CASE WHEN s.task_required = 1 THEN s.task_get_scores ELSE 0 END) AS requiredTotalScores,
-        SUM(CASE WHEN s.task_required = 0 THEN s.task_get_scores ELSE 0 END) AS electiveTotalScores
+        SUM(CASE WHEN s.task_required = 1 THEN IFNULL(s.task_get_scores, 0) ELSE 0 END) AS requiredTotalScores,
+        SUM(CASE WHEN s.task_required = 0 THEN IFNULL(s.task_get_scores, 0) ELSE 0 END) AS electiveTotalScores
         FROM
         (SELECT
         s1.stu_user_id,