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

rec/exceptions.py

@@ -74,6 +74,15 @@ class CircuitBreakerOpenError(RightmoveAPIError):
     pass
 
 
+class FloorplanDownloadError(Exception):
+    """Raised when a floorplan image download fails."""
+
+    def __init__(self, url: str, status_code: int) -> None:
+        self.url = url
+        self.status_code = status_code
+        super().__init__(f"HTTP {status_code} downloading floorplan from {url}")
+
+
 class RoutingApiError(Exception):
     """Error from the Google Routes API."""