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/MessageParser.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace DirectoryTree\ImapEngine;
+
+use ZBateson\MailMimeParser\IMessage;
+use ZBateson\MailMimeParser\MailMimeParser;
+
+class MessageParser
+{
+    /**
+     * The mail mime parser instance.
+     */
+    protected static ?MailMimeParser $parser = null;
+
+    /**
+     * Parse the given message contents.
+     */
+    public static function parse(string $contents): IMessage
+    {
+        return static::parser()->parse($contents, true);
+    }
+
+    /**
+     * Get the mail mime parser instance.
+     */
+    protected static function parser(): MailMimeParser
+    {
+        return static::$parser ??= new MailMimeParser;
+    }
+}