feat: task time estimates and velocity chart on dashboard

- Added estimatedHours column to tasks schema
- Backend: create/update support for estimatedHours
- New /api/tasks/stats/velocity endpoint: daily completions, weekly velocity, estimate totals
- Dashboard: velocity chart with 7-day bar chart, this week count, avg/week, estimate summary
- TaskDetailPanel: estimated hours input field
- CreateTaskModal: estimated hours in advanced options
- TaskCard, KanbanBoard, TaskPage: estimate badge display

frontend/src/pages/TaskPage.tsx

@@ -617,6 +617,12 @@ export function TaskPage() {
                     <span className="text-gray-700 dark:text-gray-300 font-medium">{project.name}</span>
                   </div>
                 )}
+                {task.estimatedHours != null && task.estimatedHours > 0 && (
+                  <div className="flex justify-between">
+                    <span className="text-gray-500 dark:text-gray-400">Estimate</span>
+                    <span className="text-gray-700 dark:text-gray-300 font-medium">⏱ {task.estimatedHours}h</span>
+                  </div>
+                )}
                 {task.tags?.length > 0 && (
                   <div>
                     <span className="text-gray-500 dark:text-gray-400 block mb-1">Tags</span>