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/unit/test_listing_processor.py

@@ -3,12 +3,12 @@ from datetime import datetime
 from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 from models.listing import FurnishType, ListingType
+from config.scraper_config import MAX_OCR_WORKERS
 from listing_processor import (
     _parse_furnish_type,
     _parse_available_from,
     ListingProcessor,
     FetchListingDetailsStep,
-    MAX_OCR_WORKERS,
 )
 
 
@@ -77,7 +77,7 @@ class TestListingProcessor:
         processor = ListingProcessor(mock_repo)
         for step in processor.process_steps:
             step.needs_processing = AsyncMock(return_value=True)
-            step.process = AsyncMock(side_effect=Exception("fail"))
+            step.process = AsyncMock(side_effect=ValueError("fail"))
         result = await processor.process_listing(123)
         assert result is None