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/directorytree/imapengine/composer.json

@@ -0,0 +1,46 @@
+{
+    "name": "directorytree/imapengine",
+    "type": "library",
+    "description": "A fully-featured IMAP library -- without the PHP extension",
+    "keywords": [
+        "imap",
+        "mail",
+        "engine"
+    ],
+    "homepage": "https://github.com/directorytree/imapengine",
+    "license": "MIT",
+    "authors": [
+        {
+            "name": "Steve Bauman",
+            "email": "steven_bauman@outlook.com",
+            "role": "Developer"
+        }
+    ],
+    "require": {
+        "php": "^8.1",
+        "symfony/mime": ">=6.0",
+        "nesbot/carbon": ">=2.0",
+        "illuminate/collections": ">=9.0",
+        "zbateson/mail-mime-parser": "^3.0",
+        "egulias/email-validator": "^4.0"
+    },
+    "require-dev": {
+        "spatie/ray": "^1.0",
+        "pestphp/pest": "^2.0|^3.0|^4.0"
+    },
+    "autoload": {
+        "psr-4": {
+            "DirectoryTree\\ImapEngine\\": "src"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Tests\\": "tests"
+        }
+    },
+    "config": {
+        "allow-plugins": {
+            "pestphp/pest-plugin": true
+        }
+    }
+}