fix: 评测任务软删除健壮性增强，删除不存在任务返回 404

- delete_eval_task 用 RETURNING 校验记录存在性与重复删除，否则抛 KeyError
- eval_tasks/eval_task 列表与详情过滤已软删除记录（deleted_at IS NULL）
- model_eval_delete 捕获 KeyError 返回 404，避免接口崩溃

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

backend/app/api/v1/endpoints/platform.py

@@ -2305,7 +2305,10 @@ async def model_eval_delete(task_id: str, current_user: dict = Depends(get_curre
     pending = _require_approval_or_admin("eval", task_id, current_user, f"删除评测任务 {task_id}")
     if pending:
         return pending
-    get_platform_store().delete_eval_task(task_id)
+    try:
+        get_platform_store().delete_eval_task(task_id)
+    except KeyError:
+        raise fail(404, "eval task not found")
     return ok({"deleted": task_id})