feat: add EstimatedMinutes field and general PUT update endpoint for tasks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TaskTracker.Core/DTOs/UpdateTaskRequest.cs

@@ -0,0 +1,9 @@
+namespace TaskTracker.Core.DTOs;
+
+public class UpdateTaskRequest
+{
+    public string? Title { get; set; }
+    public string? Description { get; set; }
+    public string? Category { get; set; }
+    public int? EstimatedMinutes { get; set; }
+}