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/admin.py

@@ -0,0 +1,6 @@
+from django.contrib import admin
+from .models import Tutorial, Challenge, Creation
+
+admin.site.register(Tutorial)
+admin.site.register(Challenge)
+admin.site.register(Creation)