feat: 添加批注功能

后端:
- Comment 模型（支持日记/任务/经验三种内容类型）
- CommentSerializer 和 CommentViewSet
- API: /api/comments/ - 批注 CRUD
- API: /api/comments/by_content/?content_type=diary&object_id=1 - 按内容获取批注
- 日记/任务/经验序列化器嵌套显示批注

前端:
- 批注样式（comments-section, comment-item）
- 添加批注输入框

使用方式:
- 北极星可以在任何日记/任务/经验下添加批注
- 批注会显示在内容下方
- 支持查看历史批注

backend/diary/views.py

@@ -2,8 +2,11 @@ from rest_framework import viewsets
 from rest_framework.decorators import action
 from rest_framework.response import Response
 from django.utils import timezone
-from .models import DiaryEntry, DailyProgress, Experience, Task
-from .serializers import DiaryEntrySerializer, DailyProgressSerializer, ExperienceSerializer, TaskSerializer
+from .models import DiaryEntry, DailyProgress, Experience, Task, Comment
+from .serializers import (
+    DiaryEntrySerializer, DailyProgressSerializer, ExperienceSerializer,
+    TaskSerializer, CommentSerializer
+)
 
 class DiaryEntryViewSet(viewsets.ModelViewSet):
     queryset = DiaryEntry.objects.all()
@@ -146,3 +149,24 @@ class TaskViewSet(viewsets.ModelViewSet):
         task = self.get_object()
         task.mark_completed()
         return Response(TaskSerializer(task).data)
+
+
+class CommentViewSet(viewsets.ModelViewSet):
+    queryset = Comment.objects.all()
+    serializer_class = CommentSerializer
+
+    @action(detail=False, methods=['get'])
+    def by_content(self, request):
+        """按内容类型和 ID 获取批注"""
+        content_type = request.query_params.get('content_type')
+        object_id = request.query_params.get('object_id')
+        
+        if content_type and object_id:
+            comments = Comment.objects.filter(
+                content_type=content_type,
+                object_id=object_id
+            )
+            serializer = self.get_serializer(comments, many=True)
+            return Response(serializer.data)
+        
+        return Response([])