Send each scheduled payment's due notice exactly once

The daily billing scan runs on request via WP-Cron and can overlap
itself under concurrent traffic. Each run emailed the payments it
created with no record that a notice had gone out, so two overlapping
runs could send a payer two identical "Payment due" emails for one
charge — read by families as being billed twice, though only one row
exists.

Stamp us_payments.notice_sent_at atomically before emailing: the scan
now claims each payment with a conditional UPDATE ... WHERE
notice_sent_at IS NULL and only notices, credits and batches the rows
it won. A competing run finds them claimed and stays quiet, so exactly
one notice is sent regardless of how the scan is triggered. A one-time
backfill stamps existing scheduled rows on upgrade so already-noticed
charges are not re-emailed.

Co-authored-by: anthropic/claude-opus-4-8

unsupervised-schedular.php

@@ -3,7 +3,7 @@
  * Plugin Name: Unsupervised Scheduler
  * Plugin URI:  https://git.unsupervised.ca/Unsupervised/unsupervised-scheduler
  * Description: Instructor/student lesson scheduling for WordPress.
- * Version:     1.5.7
+ * Version:     1.5.8
  * Requires at least: 6.2
  * Requires PHP: 8.1
  * Author:      Unsupervised
@@ -21,7 +21,7 @@ if (! defined('ABSPATH')) {
     exit;
 }
 
-define('USC_VERSION', '1.5.7');
+define('USC_VERSION', '1.5.8');
 define('USC_PLUGIN_FILE', __FILE__);
 define('USC_PLUGIN_DIR', plugin_dir_path(__FILE__));
 define('USC_PLUGIN_URL', plugin_dir_url(__FILE__));