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/composer/autoload_classmap.php

@@ -6,6 +6,7 @@ $vendorDir = dirname(dirname(__FILE__));
 $baseDir = dirname($vendorDir);
 
 return array(
+    'Attribute' => $vendorDir . '/symfony/polyfill-php80/Resources/stubs/Attribute.php',
     'Composer\\InstalledVersions' => $vendorDir . '/composer/InstalledVersions.php',
     'DateError' => $vendorDir . '/symfony/polyfill-php83/Resources/stubs/DateError.php',
     'DateException' => $vendorDir . '/symfony/polyfill-php83/Resources/stubs/DateException.php',
@@ -18,7 +19,12 @@ return array(
     'DateRangeError' => $vendorDir . '/symfony/polyfill-php83/Resources/stubs/DateRangeError.php',
     'Deprecated' => $vendorDir . '/symfony/polyfill-php84/Resources/stubs/Deprecated.php',
     'NoDiscard' => $vendorDir . '/symfony/polyfill-php85/Resources/stubs/NoDiscard.php',
+    'Normalizer' => $vendorDir . '/symfony/polyfill-intl-normalizer/Resources/stubs/Normalizer.php',
     'Override' => $vendorDir . '/symfony/polyfill-php83/Resources/stubs/Override.php',
+    'PhpToken' => $vendorDir . '/symfony/polyfill-php80/Resources/stubs/PhpToken.php',
     'ReflectionConstant' => $vendorDir . '/symfony/polyfill-php84/Resources/stubs/ReflectionConstant.php',
     'SQLite3Exception' => $vendorDir . '/symfony/polyfill-php83/Resources/stubs/SQLite3Exception.php',
+    'Stringable' => $vendorDir . '/symfony/polyfill-php80/Resources/stubs/Stringable.php',
+    'UnhandledMatchError' => $vendorDir . '/symfony/polyfill-php80/Resources/stubs/UnhandledMatchError.php',
+    'ValueError' => $vendorDir . '/symfony/polyfill-php80/Resources/stubs/ValueError.php',
 );