feat(03-03): integrate notifications into plugin initialization

- Added require statement for class-notifications.php in main plugin file
- Initialized notification hooks via init action in main class
- Pattern consistent with existing class initializations (Access_Control, Dashboard, etc.)
- Notifications will trigger on transition_post_status after WordPress fully loads

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

includes/class-ddhh-job-manager.php

@@ -61,5 +61,8 @@ class DDHH_JM_Job_Manager {
 
 		// Initialize access control
 		add_action( 'init', array( 'DDHH_JM_Access_Control', 'setup_hooks' ) );
+
+		// Initialize notifications
+		add_action( 'init', array( 'DDHH_JM_Notifications', 'setup_hooks' ) );
 	}
 }