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/0008_challenge_author.py

@@ -0,0 +1,28 @@
+# Generated by Codex on 2026-05-06
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ("task", "0007_taskasset"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="challenge",
+            name="author",
+            field=models.ForeignKey(
+                blank=True,
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                related_name="authored_challenges",
+                to=settings.AUTH_USER_MODEL,
+                verbose_name="出题人",
+            ),
+        ),
+    ]