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

tasks/listing_tasks.py

@@ -12,7 +12,7 @@ from config.scraper_config import ScraperConfig
 from listing_processor import ListingProcessor
 from models.listing import Listing, QueryParameters
 from rec.query import create_session, listing_query
-from rec.exceptions import CircuitBreakerOpenError, ThrottlingError
+from rec.exceptions import CircuitBreakerOpenError, InvalidResponseError, ThrottlingError
 from rec.throttle_detector import get_throttle_metrics, reset_throttle_metrics
 from repositories.listing_repository import ListingRepository
 from database import engine
@@ -324,12 +324,12 @@ async def _fetch_subquery(
                     f"Throttling on {sq.district} page {page_id}: {e}"
                 )
                 break
+            except InvalidResponseError:
+                celery_logger.debug(
+                    f"Max page for {sq.district}: {page_id - 1}"
+                )
+                break
             except Exception as e:
-                if "GENERIC_ERROR" in str(e):
-                    celery_logger.debug(
-                        f"Max page for {sq.district}: {page_id - 1}"
-                    )
-                    break
                 celery_logger.warning(
                     f"Error fetching page {page_id} for "
                     f"{sq.district}: {e}"