feat: 添加工作任务管理功能

- 新增 Task 模型（状态、优先级、进展百分比）
- 任务 API（列表、统计、进展更新、完成标记）
- 前端任务板块（统计卡片 + 任务列表）
- 进展可视化（进度条 + 进展记录）

backend/diary/serializers.py

@@ -1,5 +1,5 @@
 from rest_framework import serializers
-from .models import DiaryEntry, DailyProgress, Experience
+from .models import DiaryEntry, DailyProgress, Experience, Task
 
 class ExperienceSerializer(serializers.ModelSerializer):
     class Meta:
@@ -17,3 +17,12 @@ class DiaryEntrySerializer(serializers.ModelSerializer):
     class Meta:
         model = DiaryEntry
         fields = '__all__'
+
+class TaskSerializer(serializers.ModelSerializer):
+    status_display = serializers.CharField(source='get_status_display', read_only=True)
+    priority_display = serializers.CharField(source='get_priority_display', read_only=True)
+
+    class Meta:
+        model = Task
+        fields = '__all__'
+        read_only_fields = ['completed_at', 'created_at', 'updated_at']