add creation

自由创作：新增 Creation 模型（继承 Task，display 自动递增）与
/api/creation/ 路由，支持增删改和代码保存，只有创作者本人和超管可访问。
自由创作不进入提交/评分体系；prompt 的两个 WebSocket consumer
连接时校验 can_access_task。补充 task/tests.py 覆盖上述行为。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

task/migrations/0010_alter_task_task_type_creation.py

@@ -0,0 +1,37 @@
+# Generated by Django 6.0.1 on 2026-08-14 15:13
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('task', '0009_task_example_css_task_example_html_task_example_js'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='task',
+            name='task_type',
+            field=models.CharField(choices=[('challenge', '挑战'), ('tutorial', '教程'), ('creation', '创作')], editable=False, max_length=20, verbose_name='类型'),
+        ),
+        migrations.CreateModel(
+            name='Creation',
+            fields=[
+                ('task_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='task.task')),
+                ('html', models.TextField(blank=True, default='', verbose_name='HTML代码')),
+                ('css', models.TextField(blank=True, default='', verbose_name='CSS代码')),
+                ('js', models.TextField(blank=True, default='', verbose_name='JS代码')),
+                ('owner', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='creations', to=settings.AUTH_USER_MODEL, verbose_name='创作者')),
+            ],
+            options={
+                'verbose_name': '创作',
+                'verbose_name_plural': '创作',
+                'ordering': ('-created',),
+            },
+            bases=('task.task',),
+        ),
+    ]