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/serializers.py

@@ -1,8 +1,15 @@
 from rest_framework import serializers
-from .models import DiaryEntry, DailyProgress, Experience, Task
+from .models import DiaryEntry, DailyProgress, Experience, Task, Comment
+
+class CommentSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = Comment
+        fields = '__all__'
+        read_only_fields = ['created_at']
 
 class ExperienceSerializer(serializers.ModelSerializer):
     category_display = serializers.CharField(source='get_category_display', read_only=True)
+    comments = CommentSerializer(many=True, read_only=True, source='comment_set')
     
     class Meta:
         model = Experience
@@ -24,6 +31,7 @@ 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)
+    comments = CommentSerializer(many=True, read_only=True, source='comment_set')
     
     class Meta:
         model = DiaryEntry
@@ -33,9 +41,10 @@ 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()
+    comments = CommentSerializer(many=True, read_only=True, source='comment_set')
     
     def get_diary_entries(self, obj):
-        entries = obj.diary_entries.all()[:5]  # 最近 5 条关联日记
+        entries = obj.diary_entries.all()[:5]
         return DiaryEntrySerializer(entries, many=True).data
 
     class Meta: