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/test_listing_geojson.py

@@ -156,7 +156,7 @@ class TestStreamingEndpoint:
 
     @pytest.fixture
     def client(self):
-        """Create test client with mocked auth."""
+        """Create test client with mocked auth and rate limiting bypassed."""
         from fastapi.testclient import TestClient
         from api.app import app
         from api.auth import get_current_user, User
@@ -165,13 +165,15 @@ class TestStreamingEndpoint:
             return User(sub="test-id", email="test@example.com", name="Test User")
 
         app.dependency_overrides[get_current_user] = mock_auth
-        yield TestClient(app)
+        with patch("api.rate_limiter._match_endpoint", return_value=None):
+            yield TestClient(app)
         app.dependency_overrides.clear()
 
     @pytest.fixture
     def mock_repository(self):
-        """Mock the repository methods."""
-        with patch("api.app.ListingRepository") as MockRepo:
+        """Mock the repository methods and bypass cache."""
+        with patch("api.app.get_cached_count", return_value=None), \
+             patch("api.app.ListingRepository") as MockRepo:
             mock_instance = MagicMock()
             mock_instance.count_listings.return_value = 3
             mock_instance.stream_listings_optimized.return_value = iter([