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/egulias/email-validator/src/Validation/NoRFCWarningsValidation.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace Egulias\EmailValidator\Validation;
+
+use Egulias\EmailValidator\EmailLexer;
+use Egulias\EmailValidator\Result\InvalidEmail;
+use Egulias\EmailValidator\Result\Reason\RFCWarnings;
+
+class NoRFCWarningsValidation extends RFCValidation
+{
+    /**
+     * @var InvalidEmail|null
+     */
+    private $error;
+
+    /**
+     * {@inheritdoc}
+     */
+    public function isValid(string $email, EmailLexer $emailLexer) : bool
+    {
+        if (!parent::isValid($email, $emailLexer)) {
+            return false;
+        }
+
+        if (empty($this->getWarnings())) {
+            return true;
+        }
+
+        $this->error = new InvalidEmail(new RFCWarnings(), '');
+
+        return false;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getError() : ?InvalidEmail
+    {
+        return $this->error ?: parent::getError();
+    }
+}