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/config_file_utils.py

@@ -10,7 +10,7 @@ import tempfile
 from pathlib import Path
 from typing import cast
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.exceptions import (
     ConfigWriteError,
@@ -32,7 +32,7 @@ from app.utils.fail2ban_client import (
 from app.utils.fail2ban_response import ok, to_dict
 from app.utils.log_sanitizer import sanitize_for_logging
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # Allowlist pattern for jail names used in path construction.
 _SAFE_JAIL_NAME_RE: re.Pattern[str] = re.compile(r"^[A-Za-z0-9][A-Za-z0-9._-]{0,127}$")