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/guzzlehttp/psr7/composer.json

@@ -0,0 +1,109 @@
+{
+    "name": "guzzlehttp/psr7",
+    "description": "PSR-7 message implementation that also provides common utility methods",
+    "license": "MIT",
+    "keywords": [
+        "request",
+        "response",
+        "message",
+        "stream",
+        "http",
+        "uri",
+        "url",
+        "psr-7"
+    ],
+    "authors": [
+        {
+            "name": "Graham Campbell",
+            "email": "hello@gjcampbell.co.uk",
+            "homepage": "https://github.com/GrahamCampbell"
+        },
+        {
+            "name": "Michael Dowling",
+            "email": "mtdowling@gmail.com",
+            "homepage": "https://github.com/mtdowling"
+        },
+        {
+            "name": "George Mponos",
+            "email": "gmponos@gmail.com",
+            "homepage": "https://github.com/gmponos"
+        },
+        {
+            "name": "Tobias Nyholm",
+            "email": "tobias.nyholm@gmail.com",
+            "homepage": "https://github.com/Nyholm"
+        },
+        {
+            "name": "Márk Sági-Kazár",
+            "email": "mark.sagikazar@gmail.com",
+            "homepage": "https://github.com/sagikazarmark"
+        },
+        {
+            "name": "Tobias Schultze",
+            "email": "webmaster@tubo-world.de",
+            "homepage": "https://github.com/Tobion"
+        },
+        {
+            "name": "Márk Sági-Kazár",
+            "email": "mark.sagikazar@gmail.com",
+            "homepage": "https://sagikazarmark.hu"
+        }
+    ],
+    "require": {
+        "php": "^7.2.5 || ^8.0",
+        "psr/http-factory": "^1.0",
+        "psr/http-message": "^1.1 || ^2.0",
+        "ralouphie/getallheaders": "^3.0",
+        "symfony/deprecation-contracts": "^2.5 || ^3.0",
+        "symfony/polyfill-php80": "^1.24"
+    },
+    "require-dev": {
+        "bamarni/composer-bin-plugin": "^1.8.2",
+        "http-interop/http-factory-tests": "1.1.0",
+        "jshttp/mime-db": "1.54.0.1",
+        "phpunit/phpunit": "^8.5.52 || ^9.6.34"
+    },
+    "provide": {
+        "psr/http-factory-implementation": "1.0",
+        "psr/http-message-implementation": "1.0"
+    },
+    "suggest": {
+        "laminas/laminas-httphandlerrunner": "Emit PSR-7 responses"
+    },
+    "repositories": [
+        {
+            "type": "package",
+            "package": {
+                "name": "jshttp/mime-db",
+                "version": "1.54.0.1",
+                "dist": {
+                    "type": "zip",
+                    "url": "https://codeload.github.com/jshttp/mime-db/zip/0a9fd0bfbc87a725ff638495839114e7807b7177"
+                }
+            }
+        }
+    ],
+    "autoload": {
+        "psr-4": {
+            "GuzzleHttp\\Psr7\\": "src/"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "GuzzleHttp\\Tests\\Psr7\\": "tests/"
+        }
+    },
+    "config": {
+        "allow-plugins": {
+            "bamarni/composer-bin-plugin": true
+        },
+        "preferred-install": "dist",
+        "sort-packages": true
+    },
+    "extra": {
+        "bamarni-bin": {
+            "bin-links": true,
+            "forward-command": false
+        }
+    }
+}