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

src/NexusReader.Application/Abstractions/Services/ISanitizerService.cs

@@ -0,0 +1,12 @@
+namespace NexusReader.Application.Abstractions.Services;
+
+/// <summary>
+/// Service for sanitizing raw input text (e.g. Markdown/HTML) to protect against XSS injection.
+/// </summary>
+public interface ISanitizerService
+{
+    /// <summary>
+    /// Sanitizes the input string and returns a clean, safe version.
+    /// </summary>
+    string Sanitize(string input);
+}