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

tests/integration/test_listing_processor.py

@@ -70,7 +70,7 @@ async def test_step_failure_stops_pipeline(
     processor = ListingProcessor(listing_repository)
 
     processor.process_steps[0].needs_processing = AsyncMock(return_value=True)
-    processor.process_steps[0].process = AsyncMock(side_effect=RuntimeError("boom"))
+    processor.process_steps[0].process = AsyncMock(side_effect=ValueError("boom"))
     processor.process_steps[1].needs_processing = AsyncMock(return_value=True)
     processor.process_steps[1].process = AsyncMock()
     processor.process_steps[2].needs_processing = AsyncMock(return_value=True)