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/DroppingStream.php

@@ -0,0 +1,58 @@
+<?php
+
+declare(strict_types=1);
+
+namespace GuzzleHttp\Psr7;
+
+use Psr\Http\Message\StreamInterface;
+
+/**
+ * Stream decorator that begins dropping data once the size of the underlying
+ * stream becomes too full.
+ */
+final class DroppingStream implements StreamInterface
+{
+    use StreamDecoratorTrait;
+
+    /** @var int */
+    private $maxLength;
+
+    /** @var StreamInterface */
+    private $stream;
+
+    /**
+     * @param StreamInterface $stream    Underlying stream to decorate.
+     * @param int             $maxLength Maximum size before dropping data.
+     */
+    public function __construct(StreamInterface $stream, int $maxLength)
+    {
+        $this->stream = $stream;
+        $this->maxLength = $maxLength;
+    }
+
+    public function write($string): int
+    {
+        if (!\is_string($string)) {
+            \trigger_deprecation(
+                'guzzlehttp/psr7',
+                '2.11',
+                'Passing %s to StreamInterface::write() is deprecated; guzzlehttp/psr7 3.0 requires string for $string.',
+                \get_debug_type($string)
+            );
+        }
+
+        $diff = $this->maxLength - $this->stream->getSize();
+
+        // Begin returning 0 when the underlying stream is too large.
+        if ($diff <= 0) {
+            return 0;
+        }
+
+        // Write the stream or a subset of the stream if needed.
+        if (strlen($string) < $diff) {
+            return $this->stream->write($string);
+        }
+
+        return $this->stream->write(substr($string, 0, $diff));
+    }
+}