31. [M4] Add SMS notification channel with event subscriptions

[gravity-api]

Goal

Extend notification system with SMS channel support.

Scope

• SMS provider abstraction and configuration.
• Per-event channel subscription preferences.
• Delivery status tracking and retries.

Acceptance Criteria

• Users can receive configured events by SMS.
• Delivery failures are recorded and visible.
• Channel subscriptions are role/permission aware.