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_fetcher.py

@@ -5,7 +5,7 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 
 from models.listing import ListingType, QueryParameters
-from rec.exceptions import CircuitBreakerOpenError, ThrottlingError
+from rec.exceptions import CircuitBreakerOpenError, InvalidResponseError, ThrottlingError
 from services.listing_fetcher import (
     NUM_WORKERS,
     _fetch_subquery,
@@ -227,7 +227,7 @@ class TestFetchSubquery:
         with patch(
             "services.listing_fetcher.listing_query",
             new_callable=AsyncMock,
-            side_effect=Exception("GENERIC_ERROR: no more results"),
+            side_effect=InvalidResponseError("GENERIC_ERROR: no more results"),
         ):
             ids_found = await _fetch_subquery(
                 sq=sq,