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/guzzlehttp/psr7/src/NoSeekStream.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace GuzzleHttp\Psr7;
+
+use Psr\Http\Message\StreamInterface;
+
+/**
+ * Stream decorator that prevents a stream from being seeked.
+ */
+final class NoSeekStream implements StreamInterface
+{
+    use StreamDecoratorTrait;
+
+    /** @var StreamInterface */
+    private $stream;
+
+    public function seek($offset, $whence = SEEK_SET): void
+    {
+        if (!\is_int($offset)) {
+            \trigger_deprecation(
+                'guzzlehttp/psr7',
+                '2.11',
+                'Passing %s to StreamInterface::seek() is deprecated; guzzlehttp/psr7 3.0 requires int for $offset.',
+                \get_debug_type($offset)
+            );
+        }
+
+        if (!\is_int($whence)) {
+            \trigger_deprecation(
+                'guzzlehttp/psr7',
+                '2.11',
+                'Passing %s to StreamInterface::seek() is deprecated; guzzlehttp/psr7 3.0 requires int for $whence.',
+                \get_debug_type($whence)
+            );
+        }
+
+        throw new \RuntimeException('Cannot seek a NoSeekStream');
+    }
+
+    public function isSeekable(): bool
+    {
+        return false;
+    }
+}