Add test coverage for _serialize_notification_rule

[tmdeveloper007]

Problem

_serialize_notification_rule in backend/secuscan/routes.py serializes a DB row into a notification rule dict. No direct unit tests exist for it.

Fix

1. Extract _serialize_notification_rule into backend/secuscan/routes_notification_helpers.py.
2. Write unit tests covering all serialized fields, missing optional fields, and boolean coercion for is_active.

Acceptance Criteria

• New routes_notification_helpers.py contains the extracted function.
• Tests pass locally.