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/Connection/Result.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace DirectoryTree\ImapEngine\Connection;
+
+use DirectoryTree\ImapEngine\Collections\ResponseCollection;
+use DirectoryTree\ImapEngine\Connection\Responses\Response;
+
+class Result
+{
+    /**
+     * Constructor.
+     */
+    public function __construct(
+        protected ImapCommand $command,
+        protected array $responses = [],
+    ) {}
+
+    /**
+     * Get the executed command.
+     */
+    public function command(): ImapCommand
+    {
+        return $this->command;
+    }
+
+    /**
+     * Add a response to the result.
+     */
+    public function addResponse(Response $response): void
+    {
+        $this->responses[] = $response;
+    }
+
+    /**
+     * Get the recently received responses.
+     */
+    public function responses(): ResponseCollection
+    {
+        return new ResponseCollection($this->responses);
+    }
+}