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/composer.json

@@ -0,0 +1,37 @@
+{
+    "name": "symfony/polyfill-php80",
+    "type": "library",
+    "description": "Symfony polyfill backporting some PHP 8.0+ features to lower PHP versions",
+    "keywords": ["polyfill", "shim", "compatibility", "portable"],
+    "homepage": "https://symfony.com",
+    "license": "MIT",
+    "authors": [
+        {
+            "name": "Ion Bazan",
+            "email": "ion.bazan@gmail.com"
+        },
+        {
+            "name": "Nicolas Grekas",
+            "email": "p@tchwork.com"
+        },
+        {
+            "name": "Symfony Community",
+            "homepage": "https://symfony.com/contributors"
+        }
+    ],
+    "require": {
+        "php": ">=7.2"
+    },
+    "autoload": {
+        "psr-4": { "Symfony\\Polyfill\\Php80\\": "" },
+        "files": [ "bootstrap.php" ],
+        "classmap": [ "Resources/stubs" ]
+    },
+    "minimum-stability": "dev",
+    "extra": {
+        "thanks": {
+            "name": "symfony/polyfill",
+            "url": "https://github.com/symfony/polyfill"
+        }
+    }
+}