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/zbateson/mail-mime-parser/phpstan.neon

@@ -0,0 +1,13 @@
+parameters:
+	level: 6
+	errorFormat: raw
+	editorUrl: '%%file%% %%line%% %%column%%: %%error%%'
+	paths:
+		- src
+	bootstrapFiles:
+		- PHPStanConstants.php
+	ignoreErrors:
+		-
+			message: '#Call to an undefined method ZBateson\\MailMimeParser#'
+			paths:
+				- src/*