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/mime/composer.json

@@ -0,0 +1,45 @@
+{
+    "name": "symfony/mime",
+    "type": "library",
+    "description": "Allows manipulating MIME messages",
+    "keywords": ["mime", "mime-type"],
+    "homepage": "https://symfony.com",
+    "license": "MIT",
+    "authors": [
+        {
+            "name": "Fabien Potencier",
+            "email": "fabien@symfony.com"
+        },
+        {
+            "name": "Symfony Community",
+            "homepage": "https://symfony.com/contributors"
+        }
+    ],
+    "require": {
+        "php": ">=8.4.1",
+        "symfony/polyfill-intl-idn": "^1.10",
+        "symfony/polyfill-mbstring": "^1.0"
+    },
+    "require-dev": {
+        "egulias/email-validator": "^2.1.10|^3.1|^4",
+        "league/html-to-markdown": "^5.0",
+        "phpdocumentor/reflection-docblock": "^5.2|^6.0",
+        "symfony/dependency-injection": "^7.4|^8.0",
+        "symfony/process": "^7.4|^8.0",
+        "symfony/property-access": "^7.4|^8.0",
+        "symfony/property-info": "^7.4|^8.0",
+        "symfony/serializer": "^7.4|^8.0"
+    },
+    "conflict": {
+        "egulias/email-validator": "~3.0.0",
+        "phpdocumentor/reflection-docblock": "<5.2|>=7",
+        "phpdocumentor/type-resolver": "<1.5.1"
+    },
+    "autoload": {
+        "psr-4": { "Symfony\\Component\\Mime\\": "" },
+        "exclude-from-classmap": [
+            "/Tests/"
+        ]
+    },
+    "minimum-stability": "dev"
+}