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/src/Connection/ImapCommand.php

@@ -0,0 +1,105 @@
+<?php
+
+namespace DirectoryTree\ImapEngine\Connection;
+
+use Stringable;
+
+class ImapCommand implements Stringable
+{
+    /**
+     * The compiled command lines.
+     *
+     * @var string[]
+     */
+    protected ?array $compiled = null;
+
+    /**
+     * Constructor.
+     */
+    public function __construct(
+        protected string $tag,
+        protected string $command,
+        protected array $tokens = [],
+    ) {}
+
+    /**
+     * Get the IMAP tag.
+     */
+    public function tag(): string
+    {
+        return $this->tag;
+    }
+
+    /**
+     * Get the IMAP command.
+     */
+    public function command(): string
+    {
+        return $this->command;
+    }
+
+    /**
+     * Get the IMAP tokens.
+     */
+    public function tokens(): array
+    {
+        return $this->tokens;
+    }
+
+    /**
+     * Compile the command into lines for transmission.
+     *
+     * @return string[]
+     */
+    public function compile(): array
+    {
+        if (is_array($this->compiled)) {
+            return $this->compiled;
+        }
+
+        $lines = [];
+
+        $line = trim("{$this->tag} {$this->command}");
+
+        foreach ($this->tokens as $token) {
+            if (is_array($token)) {
+                // For tokens provided as arrays, the first element is a placeholder
+                // (for example, "{20}") that signals a literal value will follow.
+                // The second element holds the actual literal content.
+                [$placeholder, $literal] = $token;
+
+                $lines[] = "{$line} {$placeholder}";
+
+                $line = $literal;
+            } else {
+                $line .= " {$token}";
+            }
+        }
+
+        $lines[] = $line;
+
+        return $this->compiled = $lines;
+    }
+
+    /**
+     * Get a redacted version of the command for safe exposure.
+     */
+    public function redacted(): ImapCommand
+    {
+        return new static($this->tag, $this->command, array_map(
+            function (mixed $token) {
+                return is_array($token)
+                    ? array_map(fn () => '[redacted]', $token)
+                    : '[redacted]';
+            }, $this->tokens)
+        );
+    }
+
+    /**
+     * Get the command as a string.
+     */
+    public function __toString(): string
+    {
+        return implode("\r\n", $this->compile());
+    }
+}