Feat/notification throttling

[mijinummi]

Pull Request: Implement Notification Throttling

Overview

This PR implements issue #379 Implement Notification Throttling.
It prevents notification spam by adding rate limits, grouping, cooldown periods, and user controls.

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Changes Introduced

• Added notification-throttle.util.ts for throttling logic.
• Created NotificationPreferences model for user controls.
• Updated notification.service.ts to enforce throttling.
• Added /notifications/preferences endpoint for user configuration.
• Structured logs for throttling events.

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Acceptance Criteria ✅

• Rate limits enforced
• Cooldown period applied
• Grouping supported
• User controls available
• Logs record throttling events

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How to Test

1. Send >10 notifications in an hour → throttled.
2. Send notifications within cooldown → throttled.
3. Update preferences via /notifications/preferences → new limits applied.
4. Check logs → throttling events recorded.

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Contribution Notes

• Close Implement Notification Throttling #379