Add Pushover notification channel and improve status detection

Introduces Pushover as a new notification channel with priority-based alerts, device targeting, and custom sounds. Enhances domain status detection for .nl and .eu domains, ensuring accurate handling when expiration dates or explicit status flags are missing. Fixes PHP 8.x compatibility issues with null parameters in date functions and improves error handling and logging by replacing error_log() with a centralized Logger service. Updates documentation and migrations for version 1.1.1.

README.md

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 [![License](https://img.shields.io/badge/license-MIT-green.svg)](LICENSE)
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](CONTRIBUTING.md)
 
-A modern PHP MVC application for monitoring domain expiration dates and sending notifications through multiple channels (Email, Telegram, Discord, Slack). Never lose a domain again with automated monitoring and timely alerts.
+A modern PHP MVC application for monitoring domain expiration dates and sending notifications through multiple channels (Email, Telegram, Discord, Slack, Mattermost, Pushover, Webhook). Never lose a domain again with automated monitoring and timely alerts.
 
 ## ✨ Features
 
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 - 📋 **Domain Management** - Add, edit, and monitor unlimited domains
 - 🔍 **Smart WHOIS/RDAP Lookup** - Automatically fetches expiration dates and registrar information
 - 🗂️ **TLD Registry System** - Built-in support for 1,400+ TLDs with IANA integration
-- 🔔 **Multi-Channel Notifications** - Email, Telegram, Discord, and Slack support
+- 🔔 **Multi-Channel Notifications** - Email, Telegram, Discord, Slack, Mattermost, Pushover, and Webhook support
 - 👥 **Notification Groups** - Organize channels and assign domains flexibly
 - ⚡ **Real-time Dashboard** - Overview of all domains and their status
 - 📊 **Notification Logs** - Complete history of all sent notifications