feat[notifications]: Add Dokploy Backup notification type support

[WalidDevIO]

This PR adds support for the Dokploy Backup notification type across all relevant services, schemas, and UI components.

What is this PR about?

This PR introduces a new notification trigger called Dokploy Backup, which notifies users when a Dokploy system backup succeeds or fails.

The motivation behind this addition came after I noticed one of my VPS Dokploy backups had failed, but I wasn’t aware of it until several days later when manually checking the admin interface.

To prevent such cases and ensure continuous visibility over backup operations, this feature now provides both success and failure notifications through all supported channels (Slack, Telegram, Email, etc.).

Checklist

Before submitting this PR, please make sure that:

• You created a dedicated branch based on the canary branch.
• You have read the suggestions in the CONTRIBUTING.md
• You have tested this PR in your local instance.

Manual Testing

This feature has been manually tested on a local Dokploy instance, covering all main cases:

• Enabling and disabling the Dokploy Backup notification toggle for multiple providers (Slack, Telegram, Email).
• Triggering a successful backup → verified notification delivery.
• Triggering a failed backup → verified error notification delivery.
• Reloading the interface → persisted state confirmed in database.

No automated tests were added, as the notification system currently does not include test coverage and all scenarios were manually validated end-to-end.

Screenshots

Notification provider menu

Success Telegram Notification

Failed Telegram Notification