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-scheduler.php

@@ -0,0 +1,84 @@
+<?php
+/**
+ * Action Scheduler Helper Class
+ *
+ * Manages async background jobs for email batch processing.
+ *
+ * @package DDHH_Job_Manager
+ * @since 1.0.0
+ */
+
+// Exit if accessed directly.
+defined( 'ABSPATH' ) || exit;
+
+/**
+ * Class DDHH_JM_Scheduler
+ *
+ * Provides API for scheduling async email batch processing via Action Scheduler.
+ */
+class DDHH_JM_Scheduler {
+
+	/**
+	 * Initialize hooks.
+	 */
+	public static function setup_hooks() {
+		// Phase 06-03 will register async action callbacks here.
+	}
+
+	/**
+	 * Schedule mentor notification batch processing.
+	 *
+	 * Chunks user IDs into batches of 50 and schedules async actions for each batch.
+	 * This prevents email provider limits and timeouts when sending to many mentors.
+	 *
+	 * @param array $user_ids Array of user IDs to notify.
+	 * @param int   $job_id   Job post ID for the notification.
+	 * @return int Number of batches scheduled.
+	 */
+	public static function schedule_mentor_notification_batch( $user_ids, $job_id ) {
+		if ( empty( $user_ids ) || empty( $job_id ) ) {
+			error_log( 'DDHH_JM_Scheduler: Cannot schedule batch - missing user IDs or job ID' );
+			return 0;
+		}
+
+		// Chunk user IDs into batches of 50.
+		$batches = array_chunk( $user_ids, 50 );
+		$batch_count = 0;
+
+		foreach ( $batches as $batch ) {
+			// Enqueue async action for this batch.
+			// Hook: ddhh_jm_send_mentor_notification_batch
+			// Args: batch of user IDs and job ID
+			// Group: email-notifications (for organization)
+			// Unique: true (prevents duplicate scheduling)
+			// Priority: 10 (standard priority)
+			as_enqueue_async_action(
+				'ddhh_jm_send_mentor_notification_batch',
+				array(
+					'user_ids' => $batch,
+					'job_id'   => $job_id,
+				),
+				'email-notifications',
+				true,
+				10
+			);
+
+			$batch_count++;
+			error_log( sprintf(
+				'DDHH_JM_Scheduler: Scheduled batch %d with %d users for job ID %d',
+				$batch_count,
+				count( $batch ),
+				$job_id
+			) );
+		}
+
+		error_log( sprintf(
+			'DDHH_JM_Scheduler: Scheduled %d total batches for %d users for job ID %d',
+			$batch_count,
+			count( $user_ids ),
+			$job_id
+		) );
+
+		return $batch_count;
+	}
+}