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/views.py

@@ -28,11 +28,41 @@ class DiaryEntryViewSet(viewsets.ModelViewSet):
     def stats(self, request):
         """获取统计信息"""
         total_entries = DiaryEntry.objects.count()
+        total_tasks = Task.objects.count()
+        total_experiences = Experience.objects.count()
         return Response({
             'total_entries': total_entries,
             'first_entry': DiaryEntry.objects.order_by('date').first().date if total_entries > 0 else None,
             'latest_entry': DiaryEntry.objects.order_by('-date').first().date if total_entries > 0 else None,
+            'total_tasks': total_tasks,
+            'total_experiences': total_experiences,
         })
+    
+    @action(detail=True, methods=['post'])
+    def link_task(self, request, pk=None):
+        """关联任务到日记"""
+        entry = self.get_object()
+        task_id = request.data.get('task_id')
+        progress_percent = request.data.get('progress_percent')
+        notes = request.data.get('notes', '')
+        
+        task = Task.objects.get(id=task_id)
+        entry.link_task(task, progress_percent, notes)
+        
+        return Response(DiaryEntrySerializer(entry).data)
+    
+    @action(detail=True, methods=['post'])
+    def extract_experience(self, request, pk=None):
+        """从日记提炼经验"""
+        entry = self.get_object()
+        exp = entry.extract_experience(
+            title=request.data.get('title'),
+            category=request.data.get('category'),
+            problem=request.data.get('problem'),
+            solution=request.data.get('solution'),
+            lesson_learned=request.data.get('lesson_learned', '')
+        )
+        return Response(ExperienceSerializer(exp).data)
 
 class DailyProgressViewSet(viewsets.ModelViewSet):
     queryset = DailyProgress.objects.all()