fix: handle missing tenant_id in tasks service for local dev

backend/src/tasks/tasks.service.ts

@@ -14,8 +14,9 @@ export class TasksService {
     ) { }
 
     private getTenantId(): string {
+        // For local development without authentication, use default tenant
         const user = (this.request as any).user;
-        return user.tenantId;
+        return user?.tenantId || '00000000-0000-0000-0000-000000000001';
     }
 
     async findAll(projectId: string): Promise<Task[]> {