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/fail2ban_client.py

@@ -24,7 +24,7 @@ from collections.abc import Mapping, Sequence, Set
 from pathlib import Path
 from typing import TYPE_CHECKING, Protocol
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.exceptions import Fail2BanConnectionError, Fail2BanProtocolError
 
@@ -68,7 +68,7 @@ type Fail2BanResponse = tuple[int, object]
 if TYPE_CHECKING:
     from types import TracebackType
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # Attempt to reuse the vendored fail2ban package embedded in the repository.
 # If it is not on sys.path yet, load it from ``../fail2ban-master``.