Add User-Agent property to Serilog logging

Co-authored-by: marcominerva <3522534+marcominerva@users.noreply.github.com>

Author: Copilot

src/PdfSmith/Logging/HttpContextLogEventEnricher.cs

@@ -15,5 +15,10 @@ public void Enrich(LogEvent logEvent, ILogEventPropertyFactory propertyFactory)
         }
 
         logEvent.AddPropertyIfAbsent(propertyFactory.CreateProperty("UserName", httpContext.User?.Identity?.Name));
+
+        if (httpContext.Request?.Headers?.TryGetValue("User-Agent", out var userAgent) == true)
+        {
+            logEvent.AddPropertyIfAbsent(propertyFactory.CreateProperty("User-Agent", userAgent.ToString()));
+        }
     }
 }