Refactor codebase following Clean Code principles and add 229 tests

- Extract helpers to reduce function sizes (listing_tasks, app.py, query.py, listing_fetcher)
  - Replace nonlocal mutations with _PipelineState dataclass in listing_tasks
  - Fix bugs: isinstance→equality check in repository, verify_exp for OIDC tokens
  - Consolidate duplicate filter methods in listing_repository
  - Move hardcoded config to env vars with backward-compatible defaults
  - Simplify CLI decorator to auto-build QueryParameters
  - Add deprecation docstring to data_access.py
  - Test count: 158 → 387 (all passing)

crawler/rec/districts.py

@@ -36,3 +36,8 @@ def get_districts() -> dict[str, str]:
         "Wandsworth": "REGION^93977",
         "Westminster": "REGION^93980",
     }
+
+
+def get_district_by_name(name: str) -> str | None:
+    """Return the region ID for a district name, or None if not found."""
+    return get_districts().get(name)