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

config/scraper_config.py

@@ -1,10 +1,14 @@
 """Scraper configuration with environment variable loading."""
 from __future__ import annotations
 
+import multiprocessing
 import os
 from dataclasses import dataclass
 from typing import Self
 
+# Limit OCR threads to 25% of available cores to avoid starving other work.
+MAX_OCR_WORKERS = max(1, multiprocessing.cpu_count() // 4)
+
 
 @dataclass(frozen=True)
 class ScraperConfig: