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/Crypto/DkimOptions.php

@@ -0,0 +1,97 @@
+<?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\Crypto;
+
+/**
+ * A helper providing autocompletion for available DkimSigner options.
+ *
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+final class DkimOptions
+{
+    private array $options = [];
+
+    public function toArray(): array
+    {
+        return $this->options;
+    }
+
+    /**
+     * @return $this
+     */
+    public function algorithm(string $algo): static
+    {
+        $this->options['algorithm'] = $algo;
+
+        return $this;
+    }
+
+    /**
+     * @return $this
+     */
+    public function signatureExpirationDelay(int $show): static
+    {
+        $this->options['signature_expiration_delay'] = $show;
+
+        return $this;
+    }
+
+    /**
+     * @return $this
+     */
+    public function bodyMaxLength(int $max): static
+    {
+        $this->options['body_max_length'] = $max;
+
+        return $this;
+    }
+
+    /**
+     * @return $this
+     */
+    public function bodyShowLength(bool $show): static
+    {
+        $this->options['body_show_length'] = $show;
+
+        return $this;
+    }
+
+    /**
+     * @return $this
+     */
+    public function headerCanon(string $canon): static
+    {
+        $this->options['header_canon'] = $canon;
+
+        return $this;
+    }
+
+    /**
+     * @return $this
+     */
+    public function bodyCanon(string $canon): static
+    {
+        $this->options['body_canon'] = $canon;
+
+        return $this;
+    }
+
+    /**
+     * @return $this
+     */
+    public function headersToIgnore(array $headers): static
+    {
+        $this->options['headers_to_ignore'] = $headers;
+
+        return $this;
+    }
+}