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/psr/http-message/composer.json

@@ -0,0 +1,26 @@
+{
+    "name": "psr/http-message",
+    "description": "Common interface for HTTP messages",
+    "keywords": ["psr", "psr-7", "http", "http-message", "request", "response"],
+    "homepage": "https://github.com/php-fig/http-message",
+    "license": "MIT",
+    "authors": [
+        {
+            "name": "PHP-FIG",
+            "homepage": "https://www.php-fig.org/"
+        }
+    ],
+    "require": {
+        "php": "^7.2 || ^8.0"
+    },
+    "autoload": {
+        "psr-4": {
+            "Psr\\Http\\Message\\": "src/"
+        }
+    },
+    "extra": {
+        "branch-alias": {
+            "dev-master": "2.0.x-dev"
+        }
+    }
+}