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/data_access.py

@@ -1,3 +1,11 @@
+"""Legacy filesystem-based data access.
+
+.. deprecated::
+    This module is only used by the ``populate_db`` CLI command for migrating
+    old filesystem data into the database. Do not import from this module in
+    new code. Use ``models.listing.RentListing`` or ``models.listing.BuyListing``
+    and ``repositories.listing_repository.ListingRepository`` instead.
+"""
 import asyncio
 from collections import defaultdict
 from dataclasses import dataclass
@@ -381,8 +389,6 @@ class Listing:
             for item in data
         ]
 
-
-
     @property
     def listing_site(self) -> ListingSite:
         return ListingSite.RIGHTMOVE  # this class supports only right move