fix(tradein-ui): #488 BLOCK — scraper-settings response wrapper + PUT source field

deep-code-reviewer caught 2 critical bugs in both yandex/page.tsx (#488)
and cian/page.tsx (#486 already merged):

1. useScraperSettings types response as ScraperSetting[] but backend returns
   {settings: [...]} -> .find is not a function -> settings panel never renders,
   slider input never auto-populates from DB. Fix: unwrap .settings in queryFn.

2. PUT body missing required `source` field -- backend Pydantic
   ScraperSettingPayload validates source==path source. Result: 422 on every
   save. Fix: include {source, ...payload} in PUT body.

Plus M (non-blocking): useEffect race -- refetch every 30s clobbered user input
mid-edit. Guarded with initializedRef first-load-only flag.

Refs PR #488 review (deep-code-reviewer, 2026-05-23).

tradein-mvp/frontend/src/app/scrapers/yandex/page.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useEffect, useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 
 import "@/components/trade-in/trade-in.css";
@@ -17,6 +17,7 @@ interface ScraperSetting {
 }
 
 interface ScraperSettingUpdate {
+  source: string;
   request_delay_sec: number;
 }
 
@@ -79,7 +80,10 @@ interface YandexValuationTriggerResp {
 function useScraperSettings() {
   return useQuery<ScraperSetting[]>({
     queryKey: ["scraper-settings"],
-    queryFn: () => apiFetch<ScraperSetting[]>("/api/v1/admin/scraper-settings"),
+    queryFn: () =>
+      apiFetch<{ settings: ScraperSetting[] }>("/api/v1/admin/scraper-settings").then(
+        (r) => r.settings,
+      ),
     refetchInterval: 30_000,
   });
 }
@@ -89,12 +93,12 @@ function useUpdateScraperSetting() {
   return useMutation<
     ScraperSetting,
     Error,
-    { source: string; payload: ScraperSettingUpdate }
+    { source: string; payload: Omit<ScraperSettingUpdate, "source"> }
   >({
     mutationFn: ({ source, payload }) =>
       apiFetch<ScraperSetting>(`/api/v1/admin/scraper-settings/${source}`, {
         method: "PUT",
-        body: JSON.stringify(payload),
+        body: JSON.stringify({ source, ...payload }),
       }),
     onSuccess: () => {
       qc.invalidateQueries({ queryKey: ["scraper-settings"] });
@@ -212,10 +216,14 @@ export default function YandexScraperPage() {
   const settingsQ = useScraperSettings();
   const updateSettingMut = useUpdateScraperSetting();
   const [delayInput, setDelayInput] = useState<string>("5.0");
+  const delayInitializedRef = useRef(false);
 
   useEffect(() => {
     const yandex = settingsQ.data?.find((s) => s.source === "yandex");
-    if (yandex) setDelayInput(String(yandex.request_delay_sec));
+    if (yandex && !delayInitializedRef.current) {
+      setDelayInput(String(yandex.request_delay_sec));
+      delayInitializedRef.current = true;
+    }
   }, [settingsQ.data]);
 
   // Around