fix: 修复客退看板缓存命中时引用未定义常量导致 500

缓存 TTL 改成可配（DEFAULT_CACHE_TTL_MS / resolveCacheTtlMs）时漏改了
loadReplicaRows 里的比较表达式，仍写 CACHE_TTL_MS。缓存为空时短路不触发，
所以本地首轮没暴露；生产被预热任务写过缓存后，第二次请求命中该行即
ReferenceError 500。改为调用 resolveCacheTtlMs()。

backend/src/services/returnStockWarningService.js

@@ -420,7 +420,7 @@ async function loadReplicaRows({
   const basis = resolveCreatedAtBasis(createdAtBasis);
   const key = buildCacheKey(warehouse, basis);
   const cached = rowCache.get(key);
-  if (!refresh && cached && cached.rows && Date.now() - cached.at < CACHE_TTL_MS) {
+  if (!refresh && cached && cached.rows && Date.now() - cached.at < resolveCacheTtlMs()) {
     return cached.rows;
   }
   const running = inflightCache.get(key);