Add Worker background service to log periodic messages

Added a Worker class in EnvelopeGenerator.ServiceHost that logs an informational message every second. Registered the Worker as a hosted service in Program.cs to run alongside the web API.

EnvelopeGenerator.ServiceHost/Worker.cs

@@ -0,0 +1,23 @@
+namespace EnvelopeGenerator.ServiceHost;
+
+public class Worker : BackgroundService
+{
+    private readonly ILogger<Worker> _logger;
+
+    public Worker(ILogger<Worker> logger)
+    {
+        _logger = logger;
+    }
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            if (_logger.IsEnabled(LogLevel.Information))
+            {
+                _logger.LogInformation("Worker running at: {time}", DateTimeOffset.Now);
+            }
+            await Task.Delay(1000, stoppingToken);
+        }
+    }
+}