docs(06-03): complete async mentor notifications plan

Phase 6 Plan 3 complete. Async batch email notifications to opted-in
mentors on job publish without blocking admin workflow. Integrated with
Action Scheduler for background processing in batches of 50 users.
German email templates with job details, rate limit delays, error
logging, and unsubscribe hints.

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

.planning/phases/06-email-notifications/06-03-SUMMARY.md

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+# Phase 6 Plan 3: Async Mentor Notifications Summary
+
+**Async batch email notifications to opted-in mentors on job publish without blocking admin workflow**
+
+## Accomplishments
+
+- Implemented job publish hook that triggers only on initial publish (not updates), queries opted-in mentors via User Preferences class, and schedules async batch notifications via Scheduler class
+- Built async batch email processor callback that sends German email notifications with job details (title, location, type, permalink), includes rate limit delays (0.1s between emails), logs errors for failures, and provides unsubscribe hint to users
+- Integrated with existing Action Scheduler infrastructure (Phase 06-02) for background processing in batches of 50 users to prevent timeouts and email provider limits
+- Followed established email template patterns from previous phases (German language, error logging, HTML formatting with nl2br)
+
+## Files Created/Modified
+
+- `includes/class-notifications.php` - Added notify_mentors_on_job_publish() method with transition_post_status hook registration (priority 10) to trigger async batch scheduling on initial job publish
+- `includes/class-scheduler.php` - Added process_mentor_notification_batch() callback method and registered 'ddhh_jm_send_mentor_notification_batch' action hook for Action Scheduler processing
+
+## Decisions Made
+
+- Only notify on initial publish transition (pending/draft → publish), not on subsequent updates to already published posts, to avoid spam
+- Log "No opted-in mentors to notify" as informational message (not error) since empty mentor list is valid scenario
+- Use HTML email format with nl2br() for line breaks, consistent with existing notification patterns
+- Include unsubscribe hint in email body: "Um keine weiteren Benachrichtigungen zu erhalten, können Sie diese in Ihrem Profil deaktivieren"
+- Apply 0.1 second delay (usleep 100000) between emails to comply with email provider rate limits
+- Log batch completion with sent count for monitoring and debugging purposes
+
+## Issues Encountered
+
+None
+
+## Next Step
+
+Phase 6 complete. Email notification system fully functional with:
+- User opt-in preferences (Phase 06-01)
+- Action Scheduler infrastructure (Phase 06-02)
+- Async mentor notifications on job publish (Phase 06-03)
+
+Ready for Phase 7: Testing & Polish (end-to-end testing, UI refinement, deployment preparation).