chore: 将后端移动到 api/ 目录，准备合并为单仓库

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/task/migrations/0007_taskasset.py

@@ -0,0 +1,29 @@
+# Generated by Django 6.0.1 on 2026-04-13 09:33
+
+import django.db.models.deletion
+import task.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('task', '0006_challenge_pass_score'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='TaskAsset',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=100, verbose_name='文件名')),
+                ('file', models.FileField(upload_to=task.models.task_asset_upload_to, verbose_name='文件')),
+                ('task', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='assets', to='task.task')),
+            ],
+            options={
+                'verbose_name': '任务素材',
+                'verbose_name_plural': '任务素材',
+                'unique_together': {('task', 'name')},
+            },
+        ),
+    ]