Implement 'User Preference' Service for Notifications

### Context
Respecting user's focus.

### Problem
Users might want 'Yield' alerts via SMS but 'Milestone' alerts only via App.

### Suggested Execution
- Build a 'NotificationMatrix' table.
- Filter outgoing messages based on user matrix.

### Acceptance Criteria
- Granular control for the user.
- Reduced notification churn.

### References
- backend/src/notification/preferences.service.ts