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-php80/README.md

@@ -0,0 +1,25 @@
+Symfony Polyfill / Php80
+========================
+
+This component provides features added to PHP 8.0 core:
+
+- [`Stringable`](https://php.net/stringable) interface
+- [`fdiv`](https://php.net/fdiv)
+- [`ValueError`](https://php.net/valueerror) class
+- [`UnhandledMatchError`](https://php.net/unhandledmatcherror) class
+- `FILTER_VALIDATE_BOOL` constant
+- [`get_debug_type`](https://php.net/get_debug_type)
+- [`PhpToken`](https://php.net/phptoken) class
+- [`preg_last_error_msg`](https://php.net/preg_last_error_msg)
+- [`str_contains`](https://php.net/str_contains)
+- [`str_starts_with`](https://php.net/str_starts_with)
+- [`str_ends_with`](https://php.net/str_ends_with)
+- [`get_resource_id`](https://php.net/get_resource_id)
+
+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).