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/symfony/mime/Encoder/Base64MimeHeaderEncoder.php

@@ -0,0 +1,43 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Encoder;
+
+/**
+ * @author Chris Corbyn
+ */
+final class Base64MimeHeaderEncoder extends Base64Encoder implements MimeHeaderEncoderInterface
+{
+    public function getName(): string
+    {
+        return 'B';
+    }
+
+    /**
+     * Takes an unencoded string and produces a Base64 encoded string from it.
+     *
+     * If the charset is iso-2022-jp, it uses mb_encode_mimeheader instead of
+     * default encodeString, otherwise pass to the parent method.
+     */
+    public function encodeString(string $string, ?string $charset = 'utf-8', int $firstLineOffset = 0, int $maxLineLength = 0): string
+    {
+        if ('iso-2022-jp' === strtolower($charset)) {
+            $old = mb_internal_encoding();
+            mb_internal_encoding('utf-8');
+            $newstring = mb_encode_mimeheader($string, 'iso-2022-jp', $this->getName(), "\r\n");
+            mb_internal_encoding($old);
+
+            return $newstring;
+        }
+
+        return parent::encodeString($string, $charset, $firstLineOffset, $maxLineLength);
+    }
+}