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

database.py

@@ -6,10 +6,6 @@ from dotenv import load_dotenv
 load_dotenv()
 
 
-# PostgreSQL example (or use "sqlite:///database.db" for SQLite)
-# DATABASE_URL = "postgresql://user:password@localhost/db_name"
-# DATABASE_URL = "sqlite:///data/wrongmove.db"
-# DATABASE_URL = "mysql://wrongmove:wrongmove@localhost:3306/wrongmove"
 DATABASE_URL = os.environ["DB_CONNECTION_STRING"]
 
 
@@ -18,6 +14,6 @@ engine = create_engine(DATABASE_URL, echo=debug)  # `echo=True` for debug logs
 SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
 
 
-def init_db():
+def init_db() -> None:
     """Create all tables (only for development; use migrations in production)."""
     SQLModel.metadata.create_all(engine)