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/auth_service.py

@@ -13,7 +13,7 @@ import secrets
 from typing import TYPE_CHECKING
 
 import bcrypt
-import structlog
+from app.utils.logging_compat import get_logger
 
 from app.utils.async_utils import run_blocking
 
@@ -28,7 +28,7 @@ from app.repositories import settings_repo as default_settings_repo
 from app.utils.constants import SESSION_TOKEN_BYTES, SESSION_TOKEN_SIGNATURE_SEPARATOR
 from app.utils.time_utils import add_minutes, utc_now
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # Settings key for password hash
 _KEY_PASSWORD_HASH = "master_password_hash"