Rewrite email parser using ImapEngine, harden processing loop

Replace webklex/php-imap with directorytree/imapengine in the ticket
email parser. ImapEngine is pure PHP over sockets.

Parser improvements:
- Wrap per-message processing in try/catch so one malformed email
  can't abort the run; failures are flagged and logged with UID
- Query unseen + unflagged so previously-failed (flagged) messages
  are no longer re-processed on every cron run
- Skip vacation/auto-responder emails (RFC 3834) to prevent mail
  loops with the ticket auto-reply
- Cap messages per run (50) and attachment size (15MB); inline
  images over 2MB are stored as attachments instead of base64-embedded
  in ticket details
- Atomic lock file creation
- preg_quote() the ticket prefix in subject matching
- Dedupe CC watchers and exclude the sender
- Map webklex 'tls' encryption setting to STARTTLS for compatibility

NDR/DSN parsing now walks MIME parts via the underlying
zbateson parser instead of relying on attachment extraction.

plugins/vendor/composer/platform_check.php

@@ -4,8 +4,8 @@
 
 $issues = array();
 
-if (!(PHP_VERSION_ID >= 80200)) {
-    $issues[] = 'Your Composer dependencies require a PHP version ">= 8.2.0". You are running ' . PHP_VERSION . '.';
+if (!(PHP_VERSION_ID >= 80401)) {
+    $issues[] = 'Your Composer dependencies require a PHP version ">= 8.4.1". You are running ' . PHP_VERSION . '.';
 }
 
 if ($issues) {