fix(task): 加固成员归属校验，避免 family_member_id 为空时 NPE

startTask/completeTask/completeMinigameTask 三处使用 task.getFamilyMemberId().equals(memberId)，
当 family_member_id 为 NULL 时抛 NPE 并被兜底为 500，掩盖了本应返回的 400 业务提示
(如"需先完成前置任务")。改为 Objects.equals 判空安全比较。

cfc-backend/src/main/java/com/etotem/cfc/service/TaskService.java

@@ -518,7 +518,7 @@ public List<Task> getTodayTasks(Long memberId, String dimensionCode) {
     @Transactional
     public Map<String, Object> startTask(Long taskId, Long memberId) {
         Task task = taskMapper.selectById(taskId);
-        if (task == null || !task.getFamilyMemberId().equals(memberId)) {
+        if (task == null || !Objects.equals(task.getFamilyMemberId(), memberId)) {
             throw new IllegalArgumentException("任务不存在或不属于该成员");
         }
         if ("completed".equals(task.getStatus())) {
@@ -568,7 +568,7 @@ public List<Task> getTodayTasks(Long memberId, String dimensionCode) {
     @Transactional
     public Map<String, Object> completeTask(Long taskId, Long memberId, String photoUrl, String content, String contentType) {
         Task task = taskMapper.selectById(taskId);
-        if (task == null || !task.getFamilyMemberId().equals(memberId)) {
+        if (task == null || !Objects.equals(task.getFamilyMemberId(), memberId)) {
             return null;
         }
 
@@ -1205,7 +1205,7 @@ return 0; // 达到每日扣分上限
     @Transactional
     public Map<String, Object> completeMinigameTask(Long taskId, Long memberId, Integer completionTime, Integer score) {
         Task task = taskMapper.selectById(taskId);
-        if (task == null || !task.getFamilyMemberId().equals(memberId)) {
+        if (task == null || !Objects.equals(task.getFamilyMemberId(), memberId)) {
             throw new RuntimeException("任务不存在或不属于该孩子");
         }