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

backend/src/db/schema.ts

@@ -82,6 +82,7 @@ export const tasks = pgTable("tasks", {
   assigneeName: text("assignee_name"),
   projectId: uuid("project_id").references(() => projects.id, { onDelete: "set null" }),
   dueDate: timestamp("due_date", { withTimezone: true }),
+  estimatedHours: integer("estimated_hours"),
   tags: jsonb("tags").$type<string[]>().default([]),
   subtasks: jsonb("subtasks").$type<Subtask[]>().default([]),
   progressNotes: jsonb("progress_notes").$type<ProgressNote[]>().default([]),