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/FileMessage.php

@@ -0,0 +1,99 @@
+<?php
+
+namespace DirectoryTree\ImapEngine;
+
+use BackedEnum;
+use BadMethodCallException;
+
+class FileMessage implements MessageInterface
+{
+    use HasFlags, HasMessageAccessors, HasParsedMessage;
+
+    /**
+     * Constructor.
+     */
+    public function __construct(
+        protected string $contents
+    ) {}
+
+    /**
+     * {@inheritDoc}
+     */
+    public function uid(): int
+    {
+        throw new BadMethodCallException('FileMessage does not support a UID');
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function size(): ?int
+    {
+        return strlen($this->contents);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function flag(BackedEnum|string $flag, string $operation, bool $expunge = false): void
+    {
+        throw new BadMethodCallException('FileMessage does not support flagging');
+    }
+
+    /**
+     * Get the string representation of the message.
+     */
+    public function __toString(): string
+    {
+        return $this->contents;
+    }
+
+    /**
+     * Determine if this message is equal to another.
+     */
+    public function is(MessageInterface $message): bool
+    {
+        return $message instanceof self
+            && $this->contents === $message->contents;
+    }
+
+    /**
+     * Get the message flags.
+     */
+    public function flags(): array
+    {
+        return [];
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function bodyStructure(): ?BodyStructureCollection
+    {
+        return null;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function hasBodyStructure(): bool
+    {
+        return false;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function bodyPart(string $partNumber, bool $peek = true): ?string
+    {
+        throw new BadMethodCallException('FileMessage does not support fetching body parts');
+    }
+
+    /**
+     * Determine if the message is empty.
+     */
+    public function isEmpty(): bool
+    {
+        return empty($this->contents);
+    }
+}