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/app/utils/external_logging.py

@@ -16,9 +16,9 @@ from typing import TYPE_CHECKING, Any, Literal
 if TYPE_CHECKING:
     from aiohttp import ClientSession
 
-import structlog
+from app.utils.logging_compat import get_logger
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 
 class ExternalLogHandler(ABC):