[M3] Implement push notification config APIs

[rdobrik]

Description

Implement create/get/list/delete push notification config methods and provide notifier service abstraction.

Tasks

• Add push config models.
• Implement methods for create/get/list/delete.
• Add notifier interface + sample webhook notifier.
• Add retry/failure behavior and observability hooks.

Acceptance Criteria

• All push config methods are callable and validated.
• Notification retries are bounded and configurable.
• Unit tests cover create/list/delete and error cases.

Dependencies

• Issues 5, 7