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/symfony/polyfill-intl-normalizer/README.md

@@ -0,0 +1,14 @@
+Symfony Polyfill / Intl: Normalizer
+===================================
+
+This component provides a fallback implementation for the
+[`Normalizer`](https://php.net/Normalizer) class provided
+by the [Intl](https://php.net/intl) extension.
+
+More information can be found in the
+[main Polyfill README](https://github.com/symfony/polyfill/blob/main/README.md).
+
+License
+=======
+
+This library is released under the [MIT license](LICENSE).