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/laravel/serializable-closure/src/Signers/Hmac.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace Laravel\SerializableClosure\Signers;
+
+use Laravel\SerializableClosure\Contracts\Signer;
+
+class Hmac implements Signer
+{
+    /**
+     * The secret key.
+     *
+     * @var string
+     */
+    protected $secret;
+
+    /**
+     * Creates a new signer instance.
+     *
+     * @param  string  $secret
+     * @return void
+     */
+    public function __construct($secret)
+    {
+        $this->secret = $secret;
+    }
+
+    /**
+     * Sign the given serializable.
+     *
+     * @param  string  $serialized
+     * @return array
+     */
+    public function sign($serialized)
+    {
+        return [
+            'serializable' => $serialized,
+            'hash' => base64_encode(hash_hmac('sha256', $serialized, $this->secret, true)),
+        ];
+    }
+
+    /**
+     * Verify the given signature.
+     *
+     * @param  array{serializable: string, hash: string}  $signature
+     * @return bool
+     */
+    public function verify($signature)
+    {
+        return hash_equals(base64_encode(
+            hash_hmac('sha256', $signature['serializable'], $this->secret, true)
+        ), $signature['hash']);
+    }
+}