feat(06-02): create scheduler helper class for email batch actions

- Created DDHH_JM_Scheduler class with static setup_hooks() method
- Added schedule_mentor_notification_batch() method with 50-user batching
- Uses as_enqueue_async_action() with unique flag and email-notifications group
- Initialized in main plugin file and job manager class
- Ready for Phase 06-03 to register async action callbacks

includes/class-ddhh-job-manager.php

@@ -72,5 +72,11 @@ class DDHH_JM_Job_Manager {
 		if ( is_admin() ) {
 			add_action( 'init', array( 'DDHH_JM_Admin_UI', 'setup_hooks' ) );
 		}
+
+		// Initialize user preferences
+		add_action( 'init', array( 'DDHH_JM_User_Preferences', 'setup_hooks' ) );
+
+		// Initialize scheduler for async email processing
+		add_action( 'init', array( 'DDHH_JM_Scheduler', 'setup_hooks' ) );
 	}
 }