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/doctrine/lexer/composer.json

@@ -0,0 +1,55 @@
+{
+    "name": "doctrine/lexer",
+    "description": "PHP Doctrine Lexer parser library that can be used in Top-Down, Recursive Descent Parsers.",
+    "license": "MIT",
+    "type": "library",
+    "keywords": [
+        "php",
+        "parser",
+        "lexer",
+        "annotations",
+        "docblock"
+    ],
+    "authors": [
+        {
+            "name": "Guilherme Blanco",
+            "email": "guilhermeblanco@gmail.com"
+        },
+        {
+            "name": "Roman Borschel",
+            "email": "roman@code-factory.org"
+        },
+        {
+            "name": "Johannes Schmitt",
+            "email": "schmittjoh@gmail.com"
+        }
+    ],
+    "homepage": "https://www.doctrine-project.org/projects/lexer.html",
+    "require": {
+        "php": "^8.1"
+    },
+    "require-dev": {
+        "doctrine/coding-standard": "^12",
+        "phpstan/phpstan": "^1.10",
+        "phpunit/phpunit": "^10.5",
+        "psalm/plugin-phpunit": "^0.18.3",
+        "vimeo/psalm": "^5.21"
+    },
+    "autoload": {
+        "psr-4": {
+            "Doctrine\\Common\\Lexer\\": "src"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Doctrine\\Tests\\Common\\Lexer\\": "tests"
+        }
+    },
+    "config": {
+        "allow-plugins": {
+            "composer/package-versions-deprecated": true,
+            "dealerdirect/phpcodesniffer-composer-installer": true
+        },
+        "sort-packages": true
+    }
+}