修复前端长时间挂机内存累积：城市缓存 LRU 逐出 + AI 预测缓存上限

Constraint: cityDetailsByName/citySummariesByName/cityDetailMetaByName 只增不减，52 城全量可达 ~5MB+
Constraint: aiCityForecastStateCache 无上限，随城市×日期组合膨胀
Tested: tsc --noEmit OK, ruff OK, pytest 184 passed

frontend/components/dashboard/scan-terminal/ai-city-forecast-stream-state.ts

@@ -20,6 +20,16 @@ const aiCityForecastStateCache = new Map<
   string,
   { state: AiCityForecastState; updatedAt: number }
 >();
+const MAX_AI_FORECAST_CACHE_SIZE = 40;
+
+function trimAiForecastCache() {
+  if (aiCityForecastStateCache.size <= MAX_AI_FORECAST_CACHE_SIZE) return;
+  const excess = aiCityForecastStateCache.size - MAX_AI_FORECAST_CACHE_SIZE;
+  const keys = Array.from(aiCityForecastStateCache.keys());
+  for (let i = 0; i < excess && i < keys.length; i++) {
+    aiCityForecastStateCache.delete(keys[i]);
+  }
+}
 
 function isHkoObservationCity(detail?: CityDetail | null) {
   const source = String(
@@ -100,6 +110,7 @@ export function writeCachedAiForecastState(
     state,
     updatedAt: Date.now(),
   });
+  trimAiForecastCache();
 }
 
 export function readReadyCachedAiForecastState(cacheKey: string, refreshToken: number) {

frontend/hooks/useDashboardStore.tsx

@@ -659,6 +659,42 @@ export function DashboardStoreProvider({
   const [cityDetailMetaByName, setCityDetailMetaByName] = useState<
     Record<string, { cachedAt: number; revision: string }>
   >({});
+  const MAX_CACHED_CITIES = 20;
+  const cityLruRef = useRef<string[]>([]);
+
+  const touchCityLru = (cityName: string) => {
+    cityLruRef.current = [
+      ...cityLruRef.current.filter((c) => c !== cityName),
+      cityName,
+    ];
+  };
+
+  // Evict LRU cities when cache exceeds limit
+  useEffect(() => {
+    const detailsCount = Object.keys(cityDetailsByName).length;
+    if (detailsCount <= MAX_CACHED_CITIES) return;
+    const lru = cityLruRef.current;
+    const excess = detailsCount - MAX_CACHED_CITIES;
+    const toEvict = lru.slice(0, excess);
+    if (!toEvict.length) return;
+    cityLruRef.current = lru.slice(excess);
+    setCityDetailsByName((current) => {
+      const next = { ...current };
+      toEvict.forEach((c) => delete next[c]);
+      return next;
+    });
+    setCitySummariesByName((current) => {
+      const next = { ...current };
+      toEvict.forEach((c) => delete next[c]);
+      return next;
+    });
+    setCityDetailMetaByName((current) => {
+      const next = { ...current };
+      toEvict.forEach((c) => delete next[c]);
+      return next;
+    });
+  }, [cityDetailsByName]);
+
   const [selectedCity, setSelectedCity] = useState<string | null>(null);
   const [isPanelOpen, setIsPanelOpen] = useState(false);
   const [selectedForecastDate, setSelectedForecastDate] = useState<
@@ -766,6 +802,7 @@ export function DashboardStoreProvider({
   const preloadCityFromRow = (row: { city?: string | null; city_display_name?: string | null; display_name?: string | null; [key: string]: unknown }) => {
     const cityName = (row.city || row.city_display_name || row.display_name || "").trim();
     if (!cityName || findCachedCityDetail(cityDetailsByName, cityName)) return;
+    touchCityLru(cityName);
     // Pre-populate cache from scan terminal row so detail panel shows data immediately
     const now = Date.now();
     const currentTemp = Number(row.current_temp ?? row.current_max_so_far);
@@ -837,6 +874,7 @@ export function DashboardStoreProvider({
     force = false,
     depth: CityDetailDepth = "panel",
   ) => {
+    touchCityLru(cityName);
     const cached = findCachedCityDetail(cityDetailsByName, cityName);
     const cachedMeta = cityDetailMetaByName[cityName];
     const marketTargetDate =
@@ -1146,6 +1184,7 @@ export function DashboardStoreProvider({
   }, []);
 
   const ensureCitySummary = async (cityName: string, force = false) => {
+    touchCityLru(cityName);
     const existing = citySummariesRef.current[cityName];
     if (!force && existing) {
       return existing;
@@ -1346,7 +1385,9 @@ export function DashboardStoreProvider({
   const refreshAll = async () => {
     dashboardClient.clearCityDetailCache();
     setCityDetailsByName({});
+    setCitySummariesByName({});
     setCityDetailMetaByName({});
+    cityLruRef.current = [];
     if (!citiesRef.current.length) {
       await loadCities();
     }