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

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

TaskTracker.Core/Entities/WorkTask.cs

@@ -12,6 +12,7 @@ public class WorkTask
     public DateTime CreatedAt { get; set; }
     public DateTime? StartedAt { get; set; }
     public DateTime? CompletedAt { get; set; }
+    public int? EstimatedMinutes { get; set; }
 
     public int? ParentTaskId { get; set; }
     public WorkTask? ParentTask { get; set; }