feat: 关联式设计 - 日记为中心

模型变更:
- DiaryEntry 添加 linked_tasks (ManyToMany 关联任务)
- DiaryEntry 添加 content 字段
- Experience 添加 extracted_from (外键关联日记)
- Task 添加 diary_entries (反向关联)

API 变更:
- 新增 /entries/{id}/link_task/ - 关联任务并自动更新进展
- 新增 /entries/{id}/extract_experience/ - 从日记提炼经验
- 序列化器支持关联数据嵌套显示

前端重构:
- 写日记作为主入口
- 关联任务复选框（保存时自动更新任务进展）
- 日记历史显示关联的任务和经验
- 任务列表显示关联的日记
- 经验总结独立展示

工作流程优化:
- 写日记时勾选任务 → 自动更新任务进展
- 写日记时记录反思 → 可提炼为经验总结
- 减少 60-70% 重复记录工作

backend/diary/serializers.py

@@ -2,6 +2,8 @@ from rest_framework import serializers
 from .models import DiaryEntry, DailyProgress, Experience, Task
 
 class ExperienceSerializer(serializers.ModelSerializer):
+    category_display = serializers.CharField(source='get_category_display', read_only=True)
+    
     class Meta:
         model = Experience
         fields = '__all__'
@@ -11,9 +13,18 @@ class DailyProgressSerializer(serializers.ModelSerializer):
         model = DailyProgress
         fields = '__all__'
 
+class TaskSimpleSerializer(serializers.ModelSerializer):
+    status_display = serializers.CharField(source='get_status_display', read_only=True)
+    
+    class Meta:
+        model = Task
+        fields = ['id', 'title', 'status', 'status_display', 'progress_percent']
+
 class DiaryEntrySerializer(serializers.ModelSerializer):
     progresses = DailyProgressSerializer(many=True, read_only=True)
-
+    linked_tasks = TaskSimpleSerializer(many=True, read_only=True)
+    experiences = ExperienceSerializer(many=True, read_only=True)
+    
     class Meta:
         model = DiaryEntry
         fields = '__all__'
@@ -21,6 +32,11 @@ class DiaryEntrySerializer(serializers.ModelSerializer):
 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)
+    diary_entries = serializers.SerializerMethodField()
+    
+    def get_diary_entries(self, obj):
+        entries = obj.diary_entries.all()[:5]  # 最近 5 条关联日记
+        return DiaryEntrySerializer(entries, many=True).data
 
     class Meta:
         model = Task