feat(providers): add Nagios monitoring provider

[junyuc25]

Summary

Adds a Nagios monitoring provider to Keep.

• Push/webhook mode (Nagios Core + XI): Nagios notification commands send JSON payloads via curl to Keep's webhook endpoint. Works out-of-the-box for any Nagios installation.
• Pull mode (Nagios XI only): Fetches all non-OK service and host statuses from the Nagios XI REST API on demand.

Changes

• keep/providers/nagios_provider/nagios_provider.py — Full provider implementation
  • NagiosProviderAuthConfig with nagios_url, api_key, verify fields
  • _format_alert as @staticmethod handling service + host webhook payloads
  • _get_alerts pulling from Nagios XI /api/v1/objects/servicestatus and /hoststatus
  • validate_scopes verifying XI API connectivity
  • Class-level SEVERITY_MAP, STATUS_MAP, NOTIFICATION_TYPE_TO_STATUS dicts
• keep/providers/nagios_provider/ALERTS_MOCK.py — 8 sample payloads (service/host, all states, RECOVERY, ACKNOWLEDGEMENT)
• keep/providers/nagios_provider/README.md — Setup guide for webhook and pull modes, Docker-based local testing
• tests/test_nagios_provider.py — 15 unit tests (all passing)

Test plan

• _format_alert handles service CRITICAL, WARNING, UNKNOWN, and RECOVERY payloads
• _format_alert handles host DOWN, UNREACHABLE, and RECOVERY payloads
• _format_alert maps ACKNOWLEDGEMENT notification type correctly
• Severity and status maps cover all Nagios states
• Numeric state maps used for pull mode are complete
• Deterministic alert IDs are consistent across calls
• All 15 unit tests pass

Closes #3960

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[junyuc25]

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