Add process_mentor_notification_batch() method to process Action
Scheduler callbacks. Sends German email notifications with job details
(title, location, type, permalink) to batches of opted-in mentors.
Includes rate limit delay (0.1s between emails), error logging for
failures, and unsubscribe hint in email body. Registered callback for
'ddhh_jm_send_mentor_notification_batch' hook.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
- 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
