fixup(cadastre): use importlib.util.find_spec instead of import (#168 PR3)

Root cause Blocker #2: `import app.workers.tasks.scrape_cadastre` at module
top-level REBOUND `app` from FastAPI instance (line 14) to the Python package.
All subsequent `app.dependency_overrides[get_db] = ...` failed with
AttributeError because `app` was now the namespace module.

Fix: probe module availability with importlib.util.find_spec — does not
import or rebind names. FastAPI `app` instance stays intact.

All 28 cadastre tests pass locally.

backend/tests/api/v1/test_admin_cadastre.py

@@ -5,6 +5,12 @@
 
 from __future__ import annotations
 
+# scrape_cadastre зависит от app.scrapers.nspd_bulk_client (PR 2/5).
+# Используем importlib.util.find_spec вместо прямого import — иначе
+# `import app.workers...` пересоздаёт `app` как Python package и
+# перебивает FastAPI instance, привязанный в строке выше → AttributeError
+# на app.dependency_overrides.
+import importlib.util
 from typing import Any
 from unittest.mock import MagicMock, patch
 
@@ -13,13 +19,9 @@ from fastapi.testclient import TestClient
 
 from app.main import app
 
-# scrape_cadastre зависит от app.scrapers.nspd_bulk_client (PR 2/5).
-# Если ветка ещё не rebased на main — тесты которые импортируют task помечаем skip.
-_SCRAPE_CADASTRE_AVAILABLE = True
-try:
-    import app.workers.tasks.scrape_cadastre
-except ModuleNotFoundError:
-    _SCRAPE_CADASTRE_AVAILABLE = False
+_SCRAPE_CADASTRE_AVAILABLE = (
+    importlib.util.find_spec("app.workers.tasks.scrape_cadastre") is not None
+)
 
 requires_scrape_cadastre = pytest.mark.skipif(
     not _SCRAPE_CADASTRE_AVAILABLE,