enhance: Add WordPress Cron for Per-Post Email Batch Processing

[sapayth]

Summary

Fixes timeout issues when sending per-post notification emails to large subscriber lists by implementing WordPress Cron-based batch processing.

Previously, when "Send Emails" was set to "For each Post" (per-post mode), all emails were sent synchronously in a single request. This caused PHP timeouts and memory issues for sites with large subscriber lists (1000+ subscribers).

Changes

Modified publish() method

• Now collects recipients by format (plain_excerpt, plain_full, html_excerpt, html_full, public)
• Stores prepared email data (subject, bodies, sender info) in post_meta
• Schedules batch processing via cron instead of sending directly
• Preview mode unchanged - still sends immediately for testing

New methods added

Method	Purpose
schedule_per_post_email()	Stores email data in post_meta and schedules first cron batch
process_per_post_email_batch()	Cron callback that processes emails in batches
flatten_recipients()	Helper to convert grouped recipients to flat list
cleanup_email_queue()	Removes post_meta data when sending complete

Cron hook registered

• s2_process_per_post_email - triggers every 60 seconds until all emails sent

Configuration

Batch size is filterable:

add_filter( 's2_batch_size', function( $size ) {
    return 100; // Process 100 emails per batch
});