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/urls.py

@@ -1,12 +1,13 @@
 from django.urls import path, include
 from rest_framework.routers import DefaultRouter
-from .views import DiaryEntryViewSet, DailyProgressViewSet, ExperienceViewSet, TaskViewSet
+from .views import DiaryEntryViewSet, DailyProgressViewSet, ExperienceViewSet, TaskViewSet, CommentViewSet
 
 router = DefaultRouter()
 router.register(r'entries', DiaryEntryViewSet)
 router.register(r'progress', DailyProgressViewSet)
 router.register(r'experiences', ExperienceViewSet)
 router.register(r'tasks', TaskViewSet)
+router.register(r'comments', CommentViewSet)
 
 urlpatterns = [
     path('', include(router.urls)),