Add health check endpoint for Docker container monitoring

[ajlennon]

Summary

This PR adds a health check HTTP endpoint (/health) for Docker container monitoring and application status tracking.

Changes

• HealthCheck.cs: Service to track application health status including:
  • MQTT connection status
  • Last update time
  • Beacon count
  • Total nodes processed
  • Uptime
  • Version information
• HealthCheckServer.cs: HTTP server that exposes /health endpoint
  • Returns JSON health status
  • HTTP 200 for healthy, 503 for degraded
• MQTTControl.cs: Added IsConnected() method for health check
• Program.cs: Initialize and start health check server on configured port
• UWBManager.cs: Update health check metrics on each network update

Health Check Endpoint

• URL: http://localhost:{HealthCheckPort}/health
• Method: GET
• Response: JSON with health status
• Status Codes:
  • 200: Healthy (MQTT connected and recent updates)
  • 503: Degraded (MQTT disconnected or no recent updates)

Configuration

Health check port is configurable via appsettings.json:

{
  "Application": {
    "HealthCheckPort": 8080
  }
}

Example Response

{
  "status": "healthy",
  "mqttConnected": true,
  "lastUpdateTime": "2025-11-24T20:30:00Z",
  "timeSinceLastUpdate": 5.2,
  "beaconCount": 5,
  "totalNodesProcessed": 100,
  "uptimeSeconds": 3600,
  "version": "1.0.0 (Build: 2025-11-24 20:00:00 UTC, Commit: abc123)"
}

Testing

• ✅ Build succeeds
• ✅ Health check server starts on configured port
• ✅ Endpoint returns JSON health status
• ✅ Status reflects MQTT connection state
• ✅ Metrics update on network processing

Reviewer

@Jen42 - Please review this PR when you have a chance.