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

services/floorplan_detector.py

@@ -1,13 +1,10 @@
 """Floorplan detector service - OCR-based square meter detection."""
 import asyncio
+from config.scraper_config import MAX_OCR_WORKERS
 from models import Listing
 from rec import floorplan
 from repositories.listing_repository import ListingRepository
 from tqdm.asyncio import tqdm
-import multiprocessing
-
-# Use a quarter of available CPUs to avoid starving other processes
-MAX_OCR_WORKERS = max(1, multiprocessing.cpu_count() // 4)
 
 
 async def detect_floorplan(repository: ListingRepository) -> None: