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/QpMimeHeaderEncoder.php

@@ -0,0 +1,40 @@
+<?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 QpMimeHeaderEncoder extends QpEncoder implements MimeHeaderEncoderInterface
+{
+    protected function initSafeMap(): void
+    {
+        foreach (array_merge(
+            range(0x61, 0x7A), range(0x41, 0x5A),
+            range(0x30, 0x39), [0x20, 0x21, 0x2A, 0x2B, 0x2D, 0x2F]
+        ) as $byte) {
+            $this->safeMap[$byte] = \chr($byte);
+        }
+    }
+
+    public function getName(): string
+    {
+        return 'Q';
+    }
+
+    public function encodeString(string $string, ?string $charset = 'utf-8', int $firstLineOffset = 0, int $maxLineLength = 0): string
+    {
+        return str_replace([' ', '=20', "=\r\n"], ['_', '_', "\r\n"],
+            parent::encodeString($string, $charset, $firstLineOffset, $maxLineLength)
+        );
+    }
+}