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/image_fetcher.py

@@ -5,6 +5,7 @@ from pathlib import Path
 from urllib.parse import urlparse
 
 import aiohttp
+from rec.exceptions import FloorplanDownloadError
 from repositories import ListingRepository
 from tenacity import retry, stop_after_attempt, wait_random
 from tqdm.asyncio import tqdm
@@ -65,10 +66,7 @@ async def dump_images_for_listing(
                             )
                             return None
                         if response.status != 200:
-                            raise Exception(
-                                f"Error downloading floorplan for listing {listing.id} "
-                                f"from {url}: HTTP {response.status}"
-                            )
+                            raise FloorplanDownloadError(url, response.status)
                         floorplan_path.parent.mkdir(parents=True, exist_ok=True)
                         with open(floorplan_path, "wb") as f:
                             f.write(await response.read())