fix(topics): 外键约束导致 delete 400

选题有关联 content_tasks/article/publish_record/metrics 表，
外键约束阻止删除。改为先级联删除关联数据再删选题。

前端同时显示服务端返回的具体错误信息，而不是笼统的 删除失败。

platform/backend/app/api/topics.py

@@ -189,13 +189,19 @@ def delete_topic(
     if not topic:
         raise HTTPException(status_code=404, detail="Topic not found")
     _check_org(topic, current_user, db)
+    from ..models import ContentTask, Article, PublishRecord, ContentMetrics, ContentCalendar
     try:
+        db.query(ContentCalendar).filter(ContentCalendar.topic_id == topic_id).delete()
+        db.query(ContentTask).filter(ContentTask.topic_id == topic_id).delete()
+        db.query(Article).filter(Article.topic_id == topic_id).delete()
+        db.query(PublishRecord).filter(PublishRecord.topic_id == topic_id).delete()
+        db.query(ContentMetrics).filter(ContentMetrics.topic_id == topic_id).delete()
         db.delete(topic)
         db.commit()
         return {"ok": True}
     except Exception as e:
         db.rollback()
-        raise HTTPException(status_code=400, detail=f"删除失败：该选题有关联数据（文章/发布记录等），请先删除关联数据。{str(e)}")
+        raise HTTPException(status_code=400, detail=f"删除失败：{str(e)}")
 
 
 @router.post("/{topic_id}/score")