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/setup_service.py

@@ -11,7 +11,7 @@ from pathlib import Path
 from typing import TYPE_CHECKING
 
 import bcrypt
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.db import init_db, open_db
 from app.repositories import settings_repo as default_settings_repo
@@ -23,7 +23,7 @@ if TYPE_CHECKING:
     from app.repositories.protocols import SettingsRepository
     from app.services.protocols import Fail2BanMetadataService
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # Keys used in the settings table.
 _KEY_PASSWORD_HASH = "master_password_hash"