Refactor backend for cleaner error handling, DRY, and type safety

- Extract rate limiter DRY: consolidate 3 duplicated check/respond paths
  into _check_counter and _enforce_limit helpers, add proper type annotations
- Replace bare Exception raises with FloorplanDownloadError and
  RightmoveApiError; narrow catch clauses to specific exception types;
  fix Step base class to inherit from ABC
- Consolidate MAX_OCR_WORKERS into config/scraper_config.py; extract
  _find_tenure_value helper to deduplicate tenure parsing
- Extract _build_poi_distances_lookup from stream endpoint to reduce nesting
- Fix csv_exporter: optional decisions.json, NaN instead of -1 sentinels,
  guard against division by zero on missing square meters
- Fix notifications.py broken list[Surface]() constructor, database.py
  stale comments and missing type annotation, auth.py type:ignore,
  ui_exporter.py stale TODO
- Fix 3 pre-existing test failures: mock cache layer in streaming tests,
  bypass rate limiter for test isolation, fix cache invalidation test to
  account for two-pattern scan loop

tests/unit/test_listing_cache.py

@@ -204,8 +204,12 @@ class TestInvalidateCache:
         mock_client = mock.MagicMock()
         mock_pipeline = mock.MagicMock()
         mock_client.pipeline.return_value = mock_pipeline
-        # Simulate one scan iteration that returns keys, then done
-        mock_client.scan.return_value = (0, ["listings:geojson:abc", "listings:geojson:def"])
+        # invalidate_cache scans two patterns (CACHE_PREFIX*, STAGING_PREFIX*)
+        # First scan returns matching keys, second returns none
+        mock_client.scan.side_effect = [
+            (0, ["listings:geojson:abc", "listings:geojson:def"]),
+            (0, []),
+        ]
         mock_get_client.return_value = mock_client
 
         invalidate_cache()