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/migrations/0005_comment.py

@@ -0,0 +1,29 @@
+# Generated by Django 4.2.11 on 2026-04-14 11:45
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('diary', '0004_diaryentry_content_diaryentry_linked_tasks_and_more'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Comment',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('content_type', models.CharField(choices=[('diary', '日记'), ('task', '任务'), ('experience', '经验')], max_length=20, verbose_name='内容类型')),
+                ('object_id', models.IntegerField(verbose_name='内容 ID')),
+                ('content', models.TextField(verbose_name='批注内容')),
+                ('created_by', models.CharField(default='北极星', max_length=100, verbose_name='创建者')),
+                ('created_at', models.DateTimeField(auto_now_add=True, verbose_name='创建时间')),
+            ],
+            options={
+                'verbose_name': '批注',
+                'verbose_name_plural': '批注',
+                'ordering': ['-created_at'],
+            },
+        ),
+    ]