feat: add PRTG Network Monitor provider

[rothdren-lion]

Summary

Adds a new PRTG Network Monitor provider to Keep, as requested in #2282.

What does this PR do?

• Implements PrtgProvider that receives alerts from PRTG via HTTP Action (webhook) notifications
• Maps PRTG status (Up/Down/Warning/Paused/Unknown) to Keep alert status (RESOLVED/FIRING/SUPPRESSED)
• Maps PRTG priority (1-5) to Keep alert severity (INFO/WARNING/HIGH/CRITICAL)
• Provides step-by-step webhook setup instructions in webhook_markdown
• Stores PRTG-specific fields (sensor, device, group, probe, sensor_type, tags, priority) as extra attributes on AlertDto

PRTG Webhook Payload

The provider expects the following JSON payload from PRTG HTTP Action notifications (using PRTG %% placeholders):

{
  "host": "%%host",
  "name": "%%name",
  "sensor": "%%sensor",
  "message": "%%message",
  "status": "%%status",
  "lastvalue": "%%lastvalue",
  "device": "%%device",
  "group": "%%group",
  "probe": "%%probe",
  "link": "%%link",
  "id": "%%id",
  "sensorid": "%%sensorid",
  "datetime": "%%datetime",
  "down": "%%down",
  "sensor_type": "%%sensor_type",
  "tags": "%%tags",
  "priority": "%%priority"
}

Testing

• Verified _format_alert with sample PRTG alert data:
  • Down + priority 5 → CRITICAL + FIRING ✓
  • Up + priority 1 → INFO + RESOLVED ✓
  • Warning + priority 2 → WARNING + FIRING ✓

Closes #2282

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