feat: add backlog status and update task stats

- Introduced 'backlog' status in TaskStatus type and STATUS_CONFIG for better task categorization.
- Updated TaskStats interface to include backlog count.
- Enhanced getTaskStats method to fetch backlog statistics and display in Header component.
- Added backlog stat card in Header for improved visibility of task status.

components/ui/Header.tsx

@@ -90,6 +90,13 @@ export function Header({ title, subtitle, stats, syncing = false }: HeaderProps)
                 color="yellow"
               />
             )}
+            {stats.backlog > 0 && (
+              <StatCard 
+                label="BACKLOG" 
+                value={String(stats.backlog).padStart(2, '0')} 
+                color="gray"
+              />
+            )}
             {stats.todo > 0 && (
               <StatCard 
                 label="QUEUE"