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/types.ts

@@ -1,6 +1,6 @@
 // Types de base pour les tâches
 // Note: TaskStatus et TaskPriority sont maintenant gérés par la configuration centralisée dans lib/status-config.ts
-export type TaskStatus = 'todo' | 'in_progress' | 'done' | 'cancelled' | 'freeze' | 'archived';
+export type TaskStatus = 'backlog' | 'todo' | 'in_progress' | 'done' | 'cancelled' | 'freeze' | 'archived';
 export type TaskPriority = 'low' | 'medium' | 'high' | 'urgent';
 export type TaskSource = 'reminders' | 'jira' | 'manual';
 
@@ -10,6 +10,7 @@ export interface TaskStats {
   completed: number;
   inProgress: number;
   todo: number;
+  backlog: number;
   cancelled: number;
   freeze: number;
   archived: number;