Add multi-layer caching: 24h Redis TTL, stale-while-revalidate, frontend LRU cache

- Increase Redis cache TTL from 30 minutes to 24 hours
- Add stale-while-revalidate: serve stale cache (>4h) immediately while
  repopulating in background with SETNX lock to prevent concurrent rebuilds
- Add in-memory frontend LRU cache (5 entries) so repeat filter visits
  are instant without network requests
- Invalidate frontend cache on listing refresh and task completion
- Add unit tests for get_cache_age, is_cache_stale, acquire_repopulation_lock

frontend/src/services/listingCache.ts

@@ -0,0 +1,45 @@
+/**
+ * In-memory LRU cache for streaming listing results.
+ *
+ * Keyed by a deterministic hash of query parameters so that repeat visits
+ * to the same filter combination are instant (no network request).
+ */
+import type { PropertyFeature } from '@/types';
+import type { ParameterValues } from '@/components/FilterPanel';
+
+interface CacheEntry {
+  features: PropertyFeature[];
+  timestamp: number;
+}
+
+const cache = new Map<string, CacheEntry>();
+const MAX_ENTRIES = 5;
+
+export function makeCacheKey(params: ParameterValues): string {
+  const sorted = Object.entries(params)
+    .filter(([, v]) => v !== undefined && v !== null && v !== '')
+    .sort(([a], [b]) => a.localeCompare(b))
+    .map(([k, v]) => `${k}=${v instanceof Date ? v.toISOString() : v}`);
+  return sorted.join('&');
+}
+
+export function getCached(params: ParameterValues): PropertyFeature[] | null {
+  const key = makeCacheKey(params);
+  const entry = cache.get(key);
+  if (!entry) return null;
+  return entry.features;
+}
+
+export function setCached(params: ParameterValues, features: PropertyFeature[]): void {
+  const key = makeCacheKey(params);
+  if (cache.size >= MAX_ENTRIES && !cache.has(key)) {
+    // Evict oldest entry (first inserted)
+    const oldest = cache.keys().next().value;
+    if (oldest) cache.delete(oldest);
+  }
+  cache.set(key, { features, timestamp: Date.now() });
+}
+
+export function invalidateAll(): void {
+  cache.clear();
+}