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/RFCValidation.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace Egulias\EmailValidator\Validation;
+
+use Egulias\EmailValidator\EmailLexer;
+use Egulias\EmailValidator\EmailParser;
+use Egulias\EmailValidator\Result\InvalidEmail;
+use Egulias\EmailValidator\Result\Reason\ExceptionFound;
+use Egulias\EmailValidator\Warning\Warning;
+
+class RFCValidation implements EmailValidation
+{
+    /**
+     * @var Warning[]
+     */
+    private array $warnings = [];
+
+    /**
+     * @var ?InvalidEmail
+     */
+    private $error;
+
+    public function isValid(string $email, EmailLexer $emailLexer): bool
+    {
+        $parser = new EmailParser($emailLexer);
+        try {
+            $result = $parser->parse($email);
+            $this->warnings = $parser->getWarnings();
+            if ($result->isInvalid()) {
+                /** @psalm-suppress PropertyTypeCoercion */
+                $this->error = $result;
+                return false;
+            }
+        } catch (\Exception $invalid) {
+            $this->error = new InvalidEmail(new ExceptionFound($invalid), '');
+            return false;
+        }
+
+        return true;
+    }
+
+    public function getError(): ?InvalidEmail
+    {
+        return $this->error;
+    }
+
+    /**
+     * @return Warning[]
+     */
+    public function getWarnings(): array
+    {
+        return $this->warnings;
+    }
+}