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/services/config_service.py

@@ -17,7 +17,7 @@ import contextlib
 import re
 from typing import TYPE_CHECKING, TypeVar, cast
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.utils.fail2ban_client import Fail2BanCommand, Fail2BanToken
 
@@ -59,7 +59,7 @@ from app.utils.fail2ban_response import (
 )
 from app.utils.path_utils import validate_log_target
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # ---------------------------------------------------------------------------
 # Custom exceptions