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_utils/test_async_utils.py

@@ -8,7 +8,6 @@ from concurrent.futures import ThreadPoolExecutor
 from unittest import mock
 
 import pytest
-import structlog
 
 from app.utils.async_utils import logged_task, run_blocking
 
@@ -108,7 +107,7 @@ async def test_logged_task_preserves_exception_info() -> None:
     with mock.patch("app.utils.async_utils.log") as mock_log:
         await logged_task(failing_coro(), "test_task")
         mock_log.exception.assert_called_once()
-        # Verify the exception context is logged (structlog.exception captures
+        # Verify the exception context is logged (exception captures
         # the traceback automatically)
         args, kwargs = mock_log.exception.call_args
         assert args[0] == "background_task_failed"