feat: add Nagios provider with webhook support

[a638011]

Description

This PR adds a new Nagios provider that enables Keep to receive alerts from Nagios via webhook notifications.

Changes

• ✅ Implement webhook-based Nagios provider
• ✅ Support both host and service notifications
• ✅ Handle all Nagios notification types (PROBLEM, RECOVERY, ACKNOWLEDGEMENT, FLAPPING, DOWNTIME)
• ✅ Map Nagios states to Keep severity/status
• ✅ Add comprehensive documentation with setup instructions
• ✅ Include Nagios icon
• ✅ Add provider to overview page and mint.json

Implementation Details

The provider follows the same webhook pattern as other monitoring providers (Checkmk, Icinga2, LibreNMS). It:

• Accepts webhook POST requests from Nagios notification commands
• Parses both host and service alert payloads
• Generates stable alert IDs based on host/service names
• Maps Nagios states (OK/WARNING/CRITICAL/UNKNOWN/UP/DOWN/UNREACHABLE) to Keep severities
• Handles notification types for proper alert lifecycle (firing/resolved/acknowledged/suppressed)

Testing

Tested with mock alert data covering:

• Service alerts (WARNING, CRITICAL, RECOVERY)
• Host alerts (DOWN, UP, UNREACHABLE)
• Acknowledgements and downtime notifications

Documentation

Complete setup guide included in docs/providers/documentation/nagios-provider.mdx with:

• Notification command definitions for both hosts and services
• Contact and contact group configuration
• All supported Nagios macros
• Links to official Nagios documentation

Closes #3960

/claim #3960

[vercel]

@a638011 is attempting to deploy a commit to the KeepHQ Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

All committers have signed the CLA.