Add user update endpoint and DTO

Introduce ChangeUserRequest DTO (UserName, IsActive) and add UpdateAppUser action to AppUserController. The new endpoint allows updating a user's username and active state, trims and validates the username, checks for duplicates, updates the Identity security stamp when deactivating to invalidate sessions, and returns appropriate success or error responses.

API/Contracts/Auth/ChangeUserRequest.cs

@@ -0,0 +1,8 @@
+namespace API.Contracts.Auth
+{
+    public class ChangeUserRequest
+    {
+        public string? UserName { get; set; }
+        public bool? IsActive { get; set; }
+    }
+}