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/ban_service.py

@@ -16,7 +16,7 @@ import ipaddress
 from typing import TYPE_CHECKING, Any, cast
 
 import aiohttp
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.exceptions import JailNotFoundError, JailOperationError
 from app.models._common import (
@@ -69,7 +69,7 @@ if TYPE_CHECKING:
     from app.repositories.protocols import HistoryArchiveRepository
     from app.services.geo_cache import GeoCache
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 
 async def get_fail2ban_db_path(socket_path: str) -> str: