Fix duplicate listings via staged Redis cache and frontend stream cancellation

Three-pronged fix for duplicate listings appearing in the UI:

1. Backend: Replace direct rpush cache writes with staged population
   (write to temp key, then atomic RENAME to live key). Skip cache
   writes entirely for POI-enriched requests. Clean staging keys on
   invalidation.

2. Frontend: Add AbortController to cancel in-flight streaming requests
   when loadListings is called again, preventing data mixing.

3. Frontend: Deduplicate features by URL during stream accumulation as
   a safety net against any remaining server-side duplicates.

tests/integration/test_api.py

@@ -268,7 +268,9 @@ class TestStreamListingGeoJsonEndpoint:
 
         with patch("api.app.get_cached_count", return_value=None), \
              patch("api.app.ListingRepository", return_value=mock_repo), \
-             patch("api.app.cache_features_batch"):
+             patch("api.app.begin_cache_population", return_value="staging:test"), \
+             patch("api.app.cache_features_batch_staged"), \
+             patch("api.app.finalize_cache_population"):
             response = await async_client.get(
                 "/api/listing_geojson/stream",
                 params={"listing_type": "RENT"},