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/directorytree/imapengine/src/ComparesFolders.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace DirectoryTree\ImapEngine;
+
+trait ComparesFolders
+{
+    /**
+     * Determine if two folders are the same.
+     */
+    protected function isSameFolder(FolderInterface $a, FolderInterface $b): bool
+    {
+        return $a->path() === $b->path()
+            && $a->mailbox()->config('host') === $b->mailbox()->config('host')
+            && $a->mailbox()->config('username') === $b->mailbox()->config('username');
+    }
+}