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

api/app.py

@@ -1,4 +1,5 @@
 """FastAPI application for the Real Estate Crawler API."""
+import asyncio
 from datetime import datetime, timedelta
 import json
 import logging
@@ -40,6 +41,8 @@ from services.listing_cache import (
     cache_features_batch_staged,
     finalize_cache_population,
     delete_staging_key,
+    is_cache_stale,
+    acquire_repopulation_lock,
 )
 from repositories.poi_repository import POIRepository
 from repositories.decision_repository import DecisionRepository
@@ -278,6 +281,7 @@ async def _stream_from_cache(
     limit: int | None,
     user_email: str | None = None,
     decision_filter: str = "all",
+    stale: bool = False,
 ) -> AsyncGenerator[str, None]:
     """Stream GeoJSON features from the Redis cache (cache-hit path)."""
     cached_count = get_cached_count(query_parameters)
@@ -288,6 +292,7 @@ async def _stream_from_cache(
         "batch_size": batch_size,
         "total_expected": effective_total,
         "cached": True,
+        "stale": stale,
     }) + "\n"
 
     # Resolve decision IDs (deferred to after metadata is sent)
@@ -432,6 +437,30 @@ async def _stream_from_db(
             delete_staging_key(staging_key)
 
 
+async def _repopulate_cache_background(query_parameters: QueryParameters) -> None:
+    """Repopulate the cache from DB in the background (fire-and-forget)."""
+    if not acquire_repopulation_lock(query_parameters):
+        logger.debug("Skipping background repopulation — already in progress")
+        return
+    try:
+        logger.info("Starting background cache repopulation for stale entry")
+        repository = ListingRepository(engine)
+        staging_key = begin_cache_population(query_parameters)
+        try:
+            for row in repository.stream_listings_optimized(
+                query_parameters, limit=None, page_size=DEFAULT_BATCH_SIZE
+            ):
+                feature = convert_row_to_geojson(row, query_parameters.listing_type.value)
+                cache_features_batch_staged(staging_key, [feature])
+            finalize_cache_population(staging_key, query_parameters)
+            logger.info("Background cache repopulation completed")
+        except Exception:
+            delete_staging_key(staging_key)
+            raise
+    except Exception:
+        logger.exception("Background cache repopulation failed")
+
+
 @app.get("/api/listing_geojson/stream")
 async def stream_listing_geojson(
     user: Annotated[User, Depends(get_current_user)],
@@ -470,10 +499,15 @@ async def stream_listing_geojson(
 
     if cached_count is not None and cached_count > 0 and not include_poi_distances:
         app_metrics.geojson_cache_operations.add(1, {"result": "hit"})
+        stale = is_cache_stale(query_parameters)
+        if stale:
+            # Fire-and-forget background repopulation
+            asyncio.create_task(_repopulate_cache_background(query_parameters))
         generator = _stream_from_cache(
             query_parameters, batch_size, limit,
             user_email=user.email,
             decision_filter=decision_filter,
+            stale=stale,
         )
     else:
         app_metrics.geojson_cache_operations.add(1, {"result": "miss"})