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/FlaggableInterface.php

@@ -0,0 +1,127 @@
+<?php
+
+namespace DirectoryTree\ImapEngine;
+
+use BackedEnum;
+
+interface FlaggableInterface
+{
+    /**
+     * Mark the message as read. Alias for markSeen.
+     */
+    public function markRead(): void;
+
+    /**
+     * Mark the message as unread. Alias for unmarkSeen.
+     */
+    public function markUnread(): void;
+
+    /**
+     * Mark the message as seen.
+     */
+    public function markSeen(): void;
+
+    /**
+     * Unmark the seen flag.
+     */
+    public function unmarkSeen(): void;
+
+    /**
+     * Mark the message as answered.
+     */
+    public function markAnswered(): void;
+
+    /**
+     * Unmark the answered flag.
+     */
+    public function unmarkAnswered(): void;
+
+    /**
+     * Mark the message as flagged.
+     */
+    public function markFlagged(): void;
+
+    /**
+     * Unmark the flagged flag.
+     */
+    public function unmarkFlagged(): void;
+
+    /**
+     * Mark the message as deleted.
+     */
+    public function markDeleted(bool $expunge = false): void;
+
+    /**
+     * Unmark the deleted flag.
+     */
+    public function unmarkDeleted(): void;
+
+    /**
+     * Mark the message as a draft.
+     */
+    public function markDraft(): void;
+
+    /**
+     * Unmark the draft flag.
+     */
+    public function unmarkDraft(): void;
+
+    /**
+     * Mark the message as recent.
+     */
+    public function markRecent(): void;
+
+    /**
+     * Unmark the recent flag.
+     */
+    public function unmarkRecent(): void;
+
+    /**
+     * Determine if the message is marked as seen.
+     */
+    public function isSeen(): bool;
+
+    /**
+     * Determine if the message is marked as answered.
+     */
+    public function isAnswered(): bool;
+
+    /**
+     * Determine if the message is flagged.
+     */
+    public function isFlagged(): bool;
+
+    /**
+     * Determine if the message is marked as deleted.
+     */
+    public function isDeleted(): bool;
+
+    /**
+     * Determine if the message is marked as a draft.
+     */
+    public function isDraft(): bool;
+
+    /**
+     * Determine if the message is marked as recent.
+     */
+    public function isRecent(): bool;
+
+    /**
+     * Get the message's flags.
+     *
+     * @return string[]
+     */
+    public function flags(): array;
+
+    /**
+     * Determine if the message has the given flag.
+     */
+    public function hasFlag(BackedEnum|string $flag): bool;
+
+    /**
+     * Add or remove a flag from the message.
+     *
+     * @param  '+'|'-'  $operation
+     */
+    public function flag(BackedEnum|string $flag, string $operation, bool $expunge = false): void;
+}