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.

lib/status-config.ts

@@ -9,6 +9,13 @@ export interface StatusConfig {
 }
 
 export const STATUS_CONFIG: Record<TaskStatus, StatusConfig> = {
+  backlog: {
+    key: 'backlog',
+    label: 'Backlog',
+    icon: '📋',
+    color: 'gray',
+    order: 0
+  },
   todo: {
     key: 'todo',
     label: 'À faire',