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/log_service.py

@@ -9,7 +9,7 @@ import asyncio
 import re
 from pathlib import Path
 
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.exceptions import ConfigOperationError
 from app.models.config import (
@@ -29,7 +29,7 @@ from app.utils.fail2ban_client import (
 )
 from app.utils.fail2ban_response import ok
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 _NON_FILE_LOG_TARGETS: frozenset[str] = frozenset(
     {"STDOUT", "STDERR", "SYSLOG", "SYSTEMD-JOURNAL"}