Summary

Implements Nagios Provider to receive alerts via webhooks.

Features

• NagiosProviderAuthConfig for authentication
• Status/Severity mapping for Nagios states (OK, WARNING, CRITICAL, UNKNOWN, UP, DOWN, UNREACHABLE, PENDING)
• Webhook endpoint for real-time alerts
• API integration for fetching alerts from Nagios CGI

Closes #3960

/claim #3960