feat: implement stage 2 of Milkdown integration (secure upload & xss guard)

src/NexusReader.Infrastructure/Services/HtmlSanitizerService.cs

@@ -0,0 +1,30 @@
+using Ganss.Xss;
+using NexusReader.Application.Abstractions.Services;
+
+namespace NexusReader.Infrastructure.Services;
+
+/// <summary>
+/// Infrastructure implementation of ISanitizerService using the Ganss.Xss HtmlSanitizer library.
+/// </summary>
+public class HtmlSanitizerService : ISanitizerService
+{
+    private readonly HtmlSanitizer _sanitizer;
+
+    public HtmlSanitizerService()
+    {
+        _sanitizer = new HtmlSanitizer();
+        
+        // Use default configuration which is extremely secure and strips
+        // all JavaScript (script tags, onerror, onload, iframe, etc.)
+    }
+
+    public string Sanitize(string input)
+    {
+        if (string.IsNullOrEmpty(input))
+        {
+            return input;
+        }
+
+        return _sanitizer.Sanitize(input);
+    }
+}