feat: add Nagios provider via webhook notifications

[DDDDDGCSM]

Closes #3960
/claim #3960

Summary

• add a new Nagios provider for webhook-based host and service notifications
• map Nagios host/service states into Keep alert status and severity
• add Nagios provider documentation and snippet stub

Validation

• python3 -m py_compile keep/providers/nagios_provider/init.py keep/providers/nagios_provider/nagios_provider.py
• git diff --check

Notes

• this implementation focuses on the most common Nagios Core integration path: custom notification commands posting JSON payloads to Keep
• it does not add API polling, only webhook ingestion

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[shahargl]

N/A