feat: add Pushover notification provider

[pluisol]

What is this PR about?

This PR adds Pushover as a new notification provider. Pushover is a popular push notification service that supports priority levels from -2 (lowest) to 2 (emergency). For emergency priority (level 2), users can configure retry interval and expiration time.

In the Youtube video Coolify vs Dokploy: Why I decided to use one over the other by Dreams of Code, he noted that a key difference between Coolify and Dokploy is that Coolify supports Pushover while Dokploy supports Gotify. With this PR, Dokploy now supports Pushover aswell.

Changes

• Added Pushover as a new notification provider with database schema, API endpoints, and UI
• Integrated Pushover into all notification event types
• Added support for Pushover priority levels (-2 to 2) with emergency priority requiring retry/expire parameters

Checklist

• You created a dedicated branch based on the canary branch.
• You have read the suggestions in the CONTRIBUTING.md file https://github.com/Dokploy/dokploy/blob/canary/CONTRIBUTING.md#pull-request
• You have tested this PR in your local instance.
• Tested creating, updating, and deleting Pushover notifications
• Tested that notification events trigger correctly

Screenshots

• priority -2 to 1

• priority 2 (emergency: adds retry and expire)

• Test Notification received successfully on phone