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/server_service.py

@@ -12,7 +12,7 @@ from __future__ import annotations
 
 from typing import cast
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.exceptions import Fail2BanConnectionError, Fail2BanProtocolError, ServerOperationError
 from app.models.server import ServerSettingsUpdate
@@ -28,7 +28,7 @@ from app.utils.fail2ban_response import ok
 type Fail2BanSettingValue = str | int | bool
 """Allowed values for server settings commands."""
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 
 def _to_int(value: object | None, default: int) -> int: