feat(backend): implement outbound external dispatcher for notifications

[Cedarich]

Closes #117

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Implement Outbound External Dispatcher for Notifications

Description: This PR introduces a robust, asynchronous background dispatcher for sending notifications off-platform (Email/Webhooks) without blocking the main HTTP loop.

Key Changes:

• Schema Updates : Modified notification.entity.ts to track dispatch status and errors.
• Background Processing : Integrated @nestjs/schedule to run a cron job every minute that batches and processes un-dispatched notifications.
• Pluggable Senders : Added an extensible ISender interface with initial mock implementations for Email and Webhook dispatch.
• API Enhancements : Updated the Notifications controller with a test endpoint to trigger external dispatches manually.
• Stability : Resolved TypeScript strict-mode issues and updated the existing test suite in notifications.service.spec.ts .

close issue #117

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