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/query.py

@@ -10,6 +10,7 @@ from models.listing import FurnishType, ListingType
 from rec import districts
 from rec.exceptions import (
     CircuitBreakerOpenError,
+    RightmoveAPIError,
     ThrottlingError,
 )
 from rec.throttle_detector import get_throttle_metrics, validate_response
@@ -205,9 +206,9 @@ def _build_listing_params(
             7,
             14,
         ]:
-            raise Exception(
-                f"Invalid max days - {max_days_since_added} Can only be got",
-                [1, 3, 7, 14],
+            raise ValueError(
+                f"Invalid max_days_since_added={max_days_since_added}, "
+                f"must be one of [1, 3, 7, 14]"
             )
         params["maxDaysSinceAdded"] = str(max_days_since_added)
 
@@ -287,7 +288,7 @@ async def _execute_api_request(
                 )
 
                 if response.status != 200:
-                    raise Exception(
+                    raise RightmoveAPIError(
                         f"{error_context}Failed due to: {await response.text()}"
                     )