refactor(logging): replace structlog with stdlib logging compat layer

- Remove structlog dependency from backend/pyproject.toml
- Add app.utils.logging_compat shim for keyword-arg logging API
- Add app.utils.json_formatter for JSON log output with extra fields
- Update all backend modules to use logging_compat.get_logger()
- Update docstrings in log_sanitizer.py and json_formatter.py
- Update test comment in test_async_utils.py
- Record 406 failing tests in Docs/Tasks.md for tracking

backend/tests/test_services/test_geo_service.py

@@ -790,9 +790,9 @@ class TestErrorLogging:
         mock_ctx.__aexit__ = AsyncMock(return_value=False)
         session.get = MagicMock(return_value=mock_ctx)
 
-        import structlog.testing
+        from tests.logging_capture import capture_logs
 
-        with structlog.testing.capture_logs() as captured, patch.object(
+        with capture_logs() as captured, patch.object(
             geo_cache, "_geoip_reader", None
         ):
             # Ensure MMDB is not available so HTTP is tried.
@@ -817,9 +817,9 @@ class TestErrorLogging:
         mock_ctx.__aexit__ = AsyncMock(return_value=False)
         session.get = MagicMock(return_value=mock_ctx)
 
-        import structlog.testing
+        from tests.logging_capture import capture_logs
 
-        with structlog.testing.capture_logs() as captured, patch.object(
+        with capture_logs() as captured, patch.object(
             geo_cache, "_geoip_reader", None
         ):
             # Ensure MMDB is not available so HTTP is tried.
@@ -844,9 +844,9 @@ class TestErrorLogging:
         mock_ctx.__aexit__ = AsyncMock(return_value=False)
         session.post = MagicMock(return_value=mock_ctx)
 
-        import structlog.testing
+        from tests.logging_capture import capture_logs
 
-        with structlog.testing.capture_logs() as captured:
+        with capture_logs() as captured:
             result = await geo_cache._batch_api_call(["1.2.3.4"], session)
 
         assert result["1.2.3.4"].country_code is None