fix(api): quote_service 缺失时 quote-interval 兜底返回默认档位 (#261)

PUT /preferences/quote-interval 的 if not qs 分支错误调用
qs.get_min_interval()，此时 qs 必为 None，触发 AttributeError → 500。
改为返回硬编码默认值 6.0，与 GET 接口兜底写法及
QuoteService.DEFAULT_INTERVAL 保持一致，并新增复现测试。

backend/app/api/settings.py

@@ -1486,7 +1486,7 @@ def update_quote_interval(req: QuoteIntervalIn, request: Request) -> dict:
     """更新行情轮询间隔。按档位自动 clamp。"""
     qs = getattr(request.app.state, "quote_service", None)
     if not qs:
-        return {"interval": req.interval, "min_interval": qs.get_min_interval(), "max_interval": 60.0}
+        return {"interval": req.interval, "min_interval": 6.0, "max_interval": 60.0}
     clamped = qs.set_interval(req.interval)
     return {
         "interval": clamped,