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

services/listing_cache.py

@@ -15,7 +15,9 @@ logger = logging.getLogger(__name__)
 
 CACHE_PREFIX = "listings:geojson:"
 STAGING_PREFIX = "listings:geojson:staging:"
-CACHE_TTL_SECONDS = 30 * 60  # 30 minutes
+CACHE_TTL_SECONDS = 24 * 60 * 60  # 24 hours
+STALE_AFTER_SECONDS = 4 * 60 * 60  # 4 hours — serve stale, revalidate in background
+REPOPULATING_PREFIX = "listings:geojson:repopulating:"
 STAGING_TTL_SECONDS = 5 * 60  # 5 minutes safety net for orphaned staging keys
 CACHE_DB = 2
 
@@ -153,3 +155,45 @@ def invalidate_cache() -> None:
             logger.info(f"Invalidated {deleted} listing cache entries")
     except redis.RedisError as e:
         logger.warning(f"Redis cache invalidation error: {e}")
+
+
+def get_cache_age(query_params: QueryParameters) -> int | None:
+    """Return the age in seconds of a cache entry, or None if not cached."""
+    try:
+        client = _get_redis_client()
+        key = make_cache_key(query_params)
+        ttl = client.ttl(key)
+        if ttl < 0:
+            # -2 = key doesn't exist, -1 = no expiry
+            return None
+        return CACHE_TTL_SECONDS - ttl
+    except redis.RedisError as e:
+        logger.warning(f"Redis cache age check error: {e}")
+        return None
+
+
+def is_cache_stale(query_params: QueryParameters) -> bool:
+    """Return True if the cache entry exists but is older than STALE_AFTER_SECONDS."""
+    age = get_cache_age(query_params)
+    if age is None:
+        return False
+    return age > STALE_AFTER_SECONDS
+
+
+def acquire_repopulation_lock(query_params: QueryParameters) -> bool:
+    """Try to acquire a lock to prevent concurrent repopulations.
+
+    Returns True if the lock was acquired, False if another repopulation
+    is already in progress for the same query.
+    """
+    try:
+        client = _get_redis_client()
+        key = make_cache_key(query_params)
+        hash_suffix = key.removeprefix(CACHE_PREFIX)
+        lock_key = f"{REPOPULATING_PREFIX}{hash_suffix}"
+        # SETNX with 60-second TTL
+        acquired: bool = bool(client.set(lock_key, "1", nx=True, ex=60))
+        return acquired
+    except redis.RedisError as e:
+        logger.warning(f"Redis repopulation lock error: {e}")
+        return False