- Add ntfy settings (ntfyEnabled, ntfyTopic) to core types and config storage
- Create NtfyNotifier service with event-based notifications for task completion/failure
- Add Notifications section to SettingsModal for configuring ntfy topic
- Wire up NtfyNotifier in engine to send notifications on task state changes
- Include ntfy topic in task templates for per-task notification override
- Add comprehensive tests for NtfyNotifier service
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